The immune system consists of cellular and molecular components that work together to destroy antigens (Ags).

Antigen-Presenting Cells

Although some Ags can stimulate the immune response directly, T cell–dependent acquired immune responses typically require antigen-presenting cells (APCs) to present Ag-derived peptides within major histocompatibility complex (MHC) molecules. Intracellular Ag (eg, viruses) can be processed and presented to CD8 cytotoxic T cells by any nucleated cell because all nucleated cells express class I MHC molecules. By encoding proteins that interfere with this process, some viruses (eg, cytomegalovirus) can evade elimination. Extracellular Ag must be processed into peptides and complexed with surface class II MHC molecules on professional APCs to be recognized by CD4 helper T (T_H) cells. The following cells constitutively express class II MHC molecules and therefore act as professional APCs:

• B cells
• Monocytes
• Macrophages
• Dendritic cells

Monocytes in the circulation are precursors to tissue macrophages. Monocytes migrate into tissues, where over about 8 h, they develop into macrophages under the influence of macrophage colony-stimulating factor (M-CSF), secreted by various cell types (eg, endothelial cells, fibroblasts). At infection sites, activated T cells secrete cytokines (eg, interferon-γ [IFN-γ]) that induce production of macrophage migration inhibitory factor, preventing macrophages from leaving.

Macrophages are activated by IFN-γ and granulocyte-macrophage colony-stimulating factor (GM-CSF). Activated macrophages kill intracellular organisms and secrete IL-1 and tumor necrosis factor-α (TNF-α). These cytokines potentiate the secretion of IFN-γ and GM-CSF and increase the expression of adhesion molecules on endothelial cells, facilitating leukocyte influx and destruction of pathogens. Based on different gene expression profiles, subtypes of macrophages (eg, M1, M2) have been identified.

Table 1
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Macrophage Subtypes Characteristics M1 M2 Activation agent Stimulation of toll-like receptors IFN-γ (a cytokine produced by TH1 cells) IL-4 and IL-13 (cytokines produced by TH2 cells) Cytokines produced Proinflammatory cytokines (eg, TNF-α) Immunosuppressive cytokines (eg, IL-10) Other functions Promote TH1 responses Are strongly microbicidal Promote tissue remodelling IFN = interferon; IL = interleukin; TH1 cells = type 1 helper T cells; TH2 cells = type 2 helper T cells; TNF = tumor necrosis factor.

Dendritic cells are present in the skin (as Langerhans cells), lymph nodes, and tissues throughout the body. Dendritic cells in the skin act as sentinel APCs, taking up Ag, then traveling to local lymph nodes where they can activate T cells. Follicular dendritic cells are a distinct lineage, do not express class II MHC molecules, and therefore do not present Ag to T_H cells. However, they have receptors for the crystallizable fragment (Fc) region of IgG and for complement, which enable them to bind with immune complexes and present the complex to B cells in germinal centers of secondary lymphoid organs.

Polymorphonuclear Leukocytes

Polymorphonuclear (PMN) leukocytes, also called granulocytes because their cytoplasm contains granules, include

• Neutrophils
• Eosinophils
• Basophils
• Mast cells

All, except for mast cells, occur in the circulation, and all have multilobed nuclei. Mast cells are tissue-based and functionally similar to circulating blood basophils.

Neutrophils constitute 40 to 70% of total WBCs; they are a first line of defense against infection. Mature neutrophils have a half-life of about 2 to 3 days. During acute inflammatory responses (eg, to infection), neutrophils, drawn by chemotactic factors and alerted by the expression of adhesion molecules on blood vessel endothelium, leave the circulation and enter tissues. Their purpose is to phagocytose and digest pathogens. Microorganisms are killed when phagocytosis generates lytic enzymes and reactive O₂ compounds (eg, superoxide, hypochlorous acid) and triggers release of granule contents (eg, defensins, proteases, bactericidal permeability-increasing protein, lactoferrin, lysozymes). DNA and histones are also released, and they, with granule contents such as elastase, generate fibers in the surrounding tissues; the fibers may facilitate killing by trapping bacteria and focusing enzyme activity. Patients with immunodeficiencies that affect the phagocytes' ability to kill pathogens (eg, chronic granulomatous disease) are especially susceptible to chronic bacterial and fungal infections.

Eosinophils constitute up to 5% of WBCs. They target organisms too large to be engulfed; they kill by secreting toxic substances (eg, reactive O₂ compounds similar to those produced in neutrophils), major basic protein (which is toxic to parasites), eosinophil cationic protein, and several enzymes. Eosinophils are also a major source of inflammatory mediators (eg, prostaglandins, leukotrienes, platelet-activating factor, many cytokines).

Basophils constitute < 5% of WBCs and share several characteristics with mast cells, although the 2 cell types have distinct lineages. Both have high-affinity receptors for IgE called FcεRI. When these cells encounter certain Ags, the bivalent IgE molecules bound to the receptors become cross-linked, triggering cell degranulation with release of preformed inflammatory mediators (eg, histamine, platelet-activating factor) and generation of newly synthesized mediators (eg, leukotrienes, prostaglandins, thromboxanes).

Mast cells occur in different tissues of the body. Mucosal mast cell granules contain tryptase and chondroitin sulfate; connective tissue mast cell granules contain tryptase, chymase, and heparin. By releasing these mediators, mast cells play a key role in generating protective acute inflammatory responses; basophils and mast cells are the source of type I hypersensitivity reactions associated with atopic allergy (see Allergic, Autoimmune, and Other Hypersensitivity Disorders: Atopic and Allergic Disorders). Degranulation can be triggered by cross-linking of IgE receptors or by the anaphylatoxin complement fragments C3a and C5a.

Cytotoxic Leukocytes

Cytotoxic leukocytes include

• Natural killer cells
• Lymphokine-activated killers

Natural killer (NK) cells

Typical NK cells constitute 5 to 15% of peripheral blood mononuclear cells. They have a round nucleus and granular cytoplasm and induce apoptosis in infected or abnormal cells by a number of pathways. As cells of the innate response, they lack antigen-specific receptors and immunologic memory. NK cells are best characterized by CD2+, CD3-, CD4-, CD8+, CD16+ (a receptor for IgG-Fc), and CD56+ surface markers.

Typical NK cells are thought to be important for tumor surveillance. NK cells express both activating and inhibitory receptors. The activating receptors on NK cells can recognize numerous ligands on target cells (eg, MHC class I–related chain A [MICA] and chain B [MICB]); the inhibitory receptors on NK cells recognize MHC class I molecules. NK cells can kill their target only when there is no strong signal from inhibitory receptors. The presence of MHC class I molecules (normally expressed on nucleated cells) on cells therefore prevent destruction of cells; their absence indicates that the cell is infected with certain viruses that inhibit MHC expression or has lost MHC expression because cancer has changed the cell. Patients with NK-cell deficiencies (eg, some types of severe combined immunodeficiency) are especially susceptible to herpes and human papillomavirus infections.

NK cells can also secrete several cytokines (eg, IFN-γ, IL-1, TNF-α); they are a major source of IFN-γ. By secreting IFN-γ, NK cells can influence the acquired immune system by promoting differentiation of type 1 helper T (T_H1) cells and inhibiting that of type 2 (T_H2) cells.

Lymphokine-activated killers (LAK)

Some leukocytes develop into potent lymphokine-activated killers, capable of killing a wide spectrum of tumor target cells and abnormal lymphocytes (eg, infected with certain viruses). These cells are a phenomenon rather than a unique subset of cells. LAK precursors are heterogeneous but can be classified primarily as NK-like (most common) or T-cell–like.

Lymphocytes

The 2 main types of lymphocytes are

• B cells (which mature in bone marrow)
• T cells (which mature in the thymus)

They are morphologically indistinguishable but have different immune functions. They can be distinguished by Ag-specific surface receptors and molecules called clusters of differentiation (CDs), whose presence and absence define some subsets. More than 300 CDs have been identified (for further information on CD Ags, see the Human Cell Differentiation Molecules web site). Each lymphocyte recognizes a specific Ag via surface receptors.

B cells

About 5 to 15% of lymphocytes in the blood are B cells; they are also present in the spleen, lymph nodes, and mucosa-associated lymphoid tissues. B cells can present Ag to T cells, but their primary function is to develop into plasma cells, which manufacture and secrete antibodies (Abs—see Biology of the Immune System: Antibodies). Patients with B-cell immunodeficiencies (eg, X-linked agammaglobulinemia) are especially susceptible to recurrent bacterial infections.

After random rearrangement of the genes that encode immunoglobulin (Ig), B cells have the potential to recognize an almost limitless number of unique Ags. Gene rearrangement occurs in programmed steps in the bone marrow during B-cell development. The process starts with a committed stem cell, continues through pro‒B and pre‒B cell stages, and results in an immature B cell. If an immature B cell interacts with Ag, it may become inactivated (tolerant) or be eliminated (by apoptosis). Immature B cells that are not inactivated or eliminated may continue to develop into mature naive B cells, leave the marrow, and enter peripheral lymphoid organs, where they may encounter Ag. Their response to Ag has 2 stages:

• Primary immune response: When mature naive B cells first encounter Ag, they become lymphoblasts, undergo clonal proliferation, and differentiate into memory cells, which can respond to the same Ag in the future, or into mature Ab-secreting plasma cells. After first exposure, there is a latent period of days before Ab is produced. Then, only IgM is produced. After that, with the help of T cells, B cells can further rearrange their Ig genes and switch to production of IgG, IgA, or IgE. Thus, after first exposure, the response is slow and provides limited protective immunity.
• Secondary (anamnestic or booster) immune response: When memory B and T_H cells are reexposed to the Ag, the memory B cells rapidly proliferate, differentiate into mature plasma cells, and promptly produce large amounts of Ab (chiefly IgG because of a T cell–induced isotype switch). The Ab is released into the blood and other tissues, where it can react with Ag. Thus, after reexposure, the immune response is faster and more effective.

T cells

T cells develop from bone marrow stem cells that travel to the thymus, where they go through rigorous selection. There are 3 main types of T cell:

• Helper
• Regulatory
• Cytotoxic

In selection, T cells that react to self Ag presented by self MHC molecules or to self MHC molecules (regardless of the Ag presented) are eliminated by apoptosis. Only T cells that can recognize nonself Ag complexed to self MHC molecules survive; they leave the thymus for peripheral blood and lymphoid tissues. If T cells that react to self Ag are not eliminated during their development in the thymus, an autoimmune disorder can result.

Most mature T cells express either CD4 or CD8 and have an Ag-binding, Ig-like surface receptor called the T-cell receptor (TCR). Genes that encode the TCR, like Ig genes, are rearranged, resulting in defined specificity and affinity for the Ag peptide displayed in the MHC molecule of an APC. As for B cells, the number of T-cell specificities is almost limitless.

For T cells to be activated, the TCR must engage with Ag-MHC. Costimulatory accessory molecules must also interact; otherwise, the T cell becomes anergic or dies by apoptosis. Some accessory molecules (eg, CTLA-4) inhibit previously activated T cells and thus dampen the immune response. Polymorphisms in the CTLA-4 gene are associated with certain autoimmune disorders, including Graves disease and type I diabetes.

Fig. 1
Two-signal model for T-cell activation. The α and β chains of the T-cell receptor (TCR) bind to antigen (Ag)–major histocompatibility complex (MHC) on an antigen-presenting cell (APC), and CD4 or CD8 interacts with the MHC. Both actions stimulate the T cell (1st signal) through the accessory CD3 chains. However, without a 2nd (coactivation) signal, the T cell is anergic or tolerant. The TCR is structurally homologous to the B-cell receptor; the α and β (or γ and δ) chains have constant (C) and variable (V) regions. (1) = 1st signal; (2) = 2nd signal.

Helper T (T_H) cells are usually CD4 but may be CD8. They differentiate from T_H0 cells into one of the following:

• T_H1 cells: In general, T_H1 cells promote cell-mediated immunity via cytotoxic T cells and macrophages and are thus particularly involved in defense against intracellular pathogens (eg, viruses).
• T_H2 cells: T_H2 cells promote Ab production by B cells (humoral immunity) and thus are particularly involved in directing responses aimed at extracellular pathogens (eg, bacteria, parasites).
• T_H17 cells: T_H17 cells promote tissue inflammation.

Each cell type secretes several cytokines (see Table 2: Biology of the Immune System: Functions of T Cells). Different patterns of cytokine production identify other T_H-cell functional phenotypes. Depending on the stimulating pathogen, T_H1 and T_H2 cells can, to a certain extent, downregulate each other's activity, leading to dominance of a T_H1 or a T_H2 response.

Table 2
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Functions of T Cells Type Substances Produced Primary Function TH1 IFN-γ IL-2 Lymphotoxin Facilitate macrophage and cytotoxic T-cell responses TH2 IL-4 IL-5 IL-6 IL-10 IL-13 Stimulate antibody production by B cells TH17 IL-17 IL-21 IL-22 Promote inflammatory responses Regulatory TGF-β IL-10 IL-35 Suppress immune responses TC Perforin Granzymes FasL Cytokines Kill infected cells Activated NKT cells IL-4 IFN-γ May help regulate immune responses FasL = Fas ligand; IFN = interferon; IL = interleukin; NK = natural killer; TC = cytotoxic T cell; TGF = transforming growth factor; TH = helper T cell.

The distinction between the different T_H cells is clinically relevant. For example, a T_H1 response dominates in tuberculoid leprosy, and a T_H2 response dominates in lepromatous leprosy. A T_H1 response is characteristic of certain autoimmune disorders (eg, type 1 diabetes, multiple sclerosis), and a T_H2 response promotes IgE production and development of allergic disorders, as well as helps B cells produce autoantibodies in some autoimmune disorders (eg, Graves disease, myasthenia gravis). T_H17 cells, via their role in inflammation, may also contribute to autoimmune disorders such as psoriasis and RA. Patients with immunodeficiencies characterized by defective T_H17 cells (eg, hyper-IgE [Job] syndrome) are especially susceptible to infection with Candida albicans and Staphylococcus aureus.

Regulatory T cells mediate suppression of immune responses and usually express the Foxp3 transcription factor. The process involves functional subsets of CD4 or CD8 T cells that either secrete cytokines with immunosuppressive properties or suppress the immune response by poorly defined mechanisms that require cell-to-cell contact. Patients with functional mutations in Foxp3 develop the autoimmune disorder IPEX (immunodysregulation, polyendocrinopathy, enteropathy, X-linked) syndrome.

Cytotoxic T (T_C) cells are usually CD8 but may be CD4; they are vital for eliminating intracellular pathogens, especially viruses. T_C cells play a role in organ transplant rejection.

T_C-cell development involves 3 phases:

• A precursor cell that, when appropriately stimulated, can differentiate into a T_C cell
• An effector cell that has differentiated and can kill its appropriate target
• A memory cell that is quiescent (no longer stimulated) but is ready to become an effector when restimulated by the original Ag-MHC combination

Fully activated T_C cells, like NK cells, can kill an infected target cell by inducing apoptosis.

T_C cells may be

• Syngeneic: Generated in response to self (autologous) cells modified by viral infection or other foreign proteins
• Allogeneic: Generated in response to cells that express foreign MHC products (eg, in organ transplantation when the donor's MHC molecules differ from the recipient's)

Some T_C cells can directly recognize foreign MHC (direct pathway); others may recognize fragments of foreign MHC presented by self MHC molecules of the transplant recipient (indirect pathway).

NKT cells are a distinct subset of T cells. Activated NKT cells secrete IL-4 and IFN-γ and may help regulate immune responses.

Antibodies

Abs act as the Ag receptor on the surface of B cells and, in response to Ag, are subsequently secreted by plasma cells. Abs recognize specific configurations (epitopes, or antigenic determinants) on the surfaces of Ags (eg, proteins, polysaccharides, nucleic acids). Abs and Ags fit tightly together because their shape and other surface properties (eg, charge) are complementary. The same Ab molecule can cross-react with related Ags if their epitopes are similar enough to those of the original Ag.

Structure

Abs consist of 4 polypeptide chains (2 identical heavy chains and 2 identical light chains) joined by disulfide bonds to produce a Y configuration (see Fig. 2: Biology of the Immune System: B-cell receptor.). The heavy and light chains are divided into a variable (V) region and a constant (C) region.

Fig. 2
B-cell receptor. The B-cell receptor consists of an Ig molecule anchored to the cell's surface. CH = heavy chain constant region; CL = light chain constant region; Fab = antigen-binding fragment; Fc = crystallizable fragment; VH = heavy chain variable region; VL = light chain variable region.

V regions are located at the amino-terminal ends of the Y arms; they are called variable because the amino acids they contain are different in different Abs. The amino acids present determine the specificity of the Ig. Hypervariable regions within the V regions contain idiotypic determinants, to which certain natural (anti-idiotype) Abs can bind; this binding may help regulate B-cell responses. A B cell can switch the Ig heavy chain isotype it produces, but it retains its heavy chain V region and the entire light-chain, thereby retaining antigenic specificity.

The C region contains a relatively constant sequence of amino acids that is distinctive for each Ig isotype.

The amino-terminal (variable) end of the Ab binds to Ag to form an Ab-Ag complex. The Ag-binding (Fab) portion of Ig consists of a light chain and a fragment of a heavy chain and contains the V region of the Ig molecule (ie, the combining sites). The crystallizable fragment (Fc) contains most of the C region of the heavy chains; Fc is responsible for complement activation and binds to Fc receptors on cells.

Antibody classes

Antibodies are divided into 5 classes:

• IgM
• IgG
• IgA
• IgD
• IgE

The classes are defined by their type of heavy chain (μ for IgM, γ for IgG, α for IgA, ε for IgE, and δ for IgD); there are also 2 types of light chains (κ and λ). Each of the 5 Ig classes can bear either κ or λ light chains.

IgM is the first Ab formed after exposure to new Ag. It has 5 Y-shaped molecules (10 heavy chains and 10 light chains), linked by a single joining (J) chain. IgM circulates primarily in the intravascular space; it complexes with and agglutinates Ag and can activate complement, thereby facilitating phagocytosis. Isohemagglutinins and many Abs to gram-negative bacteria are IgM. Monomeric IgM acts as a surface Ag receptor on B cells. Patients with hyper-IgM syndrome have a defect in the genes involved in antibody class switching (eg, genes that encode CD40 or CD154); therefore, IgA, IgG, and IgE levels are low or absent, and levels of circulating IgM are often high.

IgG is the most prevalent Ig isotype in serum and is also present in intravascular and extravascular spaces. It coats Ag to activate complement and facilitate phagocytosis by neutrophils and macrophages. IgG is the primary circulating Ig produced after reexposure to Ag (secondary immune response) and is the predominant isotype contained in commercial γ-globulin products. IgG protects against bacteria, viruses, and toxins; it is the only Ig isotype that crosses the placenta. Therefore, this class of antibody is important for protecting neonates, but pathogenic IgG antibodies (eg, anti-Rh₀[D] antibodies, stimulatory anti-TSH receptor autoantibodies), if present in the mother, can potentially cause significant disease in the fetus.

There are 4 subclasses of IgG: IgG1, IgG2, IgG3, and IgG4. They are numbered in descending order of serum concentration. IgG subclasses differ functionally mainly in their ability to activate complement; IgG1 and IgG3 are most efficient, IgG2 is less efficient, and IgG4 is inefficient. IgG1 and IgG3 are efficient mediators of Ab-dependent cellular cytotoxicity; IgG4 and IgG2 are less so.

IgA occurs at mucosal surfaces, in serum, and in secretions (saliva; tears; respiratory, GU, and GI tract secretions; colostrum), where it provides an early antibacterial and antiviral defense. J chain links IgA into a dimer to form secretory IgA. Secretory IgA is synthesized by plasma cells in the subepithelial regions of the GI and respiratory tracts. Selective IgA deficiency is relatively common but often has little clinical impact because there is cross-functionality with other classes of antibody.

IgD is coexpressed with IgM on the surface of naive B cells. Whether these 2 classes function differently on the surface of the B cell and, if so, how differently are unclear. They may simply be an example of molecular degeneracy. Serum IgD levels are very low, and the function of circulating IgD is unknown.

IgE is present in low levels in serum and in respiratory and GI mucous secretions. IgE binds with high affinity to receptors present in high levels on mast cells and basophils and to a lesser extent on several other hematopoietic cells, including dendritic cells. If Ag bridges 2 IgE molecules bound to the mast cell or basophil surface, the cells degranulate, releasing chemical mediators that cause an inflammatory response. IgE levels are elevated in atopic disorders (eg, allergic or extrinsic asthma, hay fever, atopic dermatitis) and parasitic infections.

Acute Phase Reactants

Acute phase reactants are plasma proteins whose levels dramatically increase (called positive acute phase reactants) or, in some cases, decrease (called negative acute phase reactants) if infection or tissue damage occurs. Most dramatically increased are C-reactive protein (CRP) and mannose-binding lectin (which fix complement and act as opsonins), the transport protein α₁-acid glycoprotein, and serum amyloid P component. CRP and ESR are often measured; elevated levels are a nonspecific indicator suggesting infection or inflammation. Increased fibrinogen is the main reason ESR is elevated.

Many acute phase reactants are made in the liver. Collectively, they may help limit tissue injury, enhance host resistance to infection, and promote tissue repair and resolution of inflammation.

Cytokines

Cytokines are polypeptides secreted by immune and other cells when the cell interacts with a specific Ag, with pathogen-associated molecules such as endotoxin, or with other cytokines. Main categories include

• IFNs (IFN-α, IFN-β, IFN-γ)
• TNFs (TNF-α, lymphotoxin-α, lymphotoxin-β)
• ILs
• Chemokines
• TGFs
• Hematopoietic colony-stimulating factors (CSFs)

Although lymphocyte interaction with a specific Ag triggers cytokine secretion, cytokines themselves are not Ag-specific; thus, they bridge innate and acquired immunity and generally influence the magnitude of inflammatory or immune responses. They act sequentially, synergistically, or antagonistically. They may act in an autocrine or paracrine manner.

Table 3
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Selected Cytokines Cytokine Major Sources Main Effects Interleukins (IL) IL-1α IL-1β B cells, dendritic cells, endothelium, macrophages, monocytes, NK cells Costimulates T-cell activation by enhancing production of cytokines (eg, IL-2 and its receptor) Enhances B-cell proliferation and maturation Enhances NK-cell cytotoxicity Induces IL-1, IL-6, IL-8, TNF, GM-CSF, and prostaglandin E2production by macrophages Is proinflammatory by inducing chemokines, ICAM-1, and VCAM-1 on endothelium Induces sleep, anorexia, release of tissue factor, acute phase reactants, and bone resorption by osteoclasts Is an endogenous pyrogen IL-2 TH1 cells Induces proliferation of activated T and B cells Enhances NK-cell cytotoxicity and killing of tumor cells and bacteria by monocytes and macrophages IL-4 Mast cells, NK cells, NKT cells, γδ T cells, TC2 cells, TH2 cells Induces TH2 cells Stimulates proliferation of activated B, T, and mast cells Upregulates class II MHC molecules on B cells and on macrophages and CD23 on B cells Downregulates IL-12 production and thereby inhibits TH1 cell-differentiation Increases macrophage phagocytosis Induces switch to IgG1 and IgE IL-5 Mast cells, TH2 cells Induces proliferation of eosinophils and activated B cells Induces switch to IgA IL-6 Dendritic cells, fibroblasts, macrophages, monocytes, TH2 cells Induces differentiation of B cells into plasma cells and differentiation of myeloid stem cells Induces acute phase reactants Enhances T-cell proliferation Induces TC-cell differentiation Is a pyrogen IL-7 Bone marrow and thymus stromal cells Induces differentiation of lymphoid stem cells into T- and B-cell precursors Activates mature T cells IL-8 (chemokine) Endothelial cells, macrophages, monocytes Mediates chemotaxis and activation of neutrophils IL-9 TH cells Induces proliferation of thymocytes Enhances mast cell growth Acts synergistically with IL-4 to induce switch to IgG1 and IgE IL-10 B cells, macrophages, monocytes, TC cells, TH2 cells, regulatory T cells Inhibits IL-2 secretion by human TH1 cells Downregulates production of class II MHC molecules and cytokines (eg, IL-12) by monocytes, macrophages, and dendritic cells and thereby inhibits TH1-cell differentiation Inhibits T-cell proliferation Enhances B-cell differentiation IL-12 B cells, dendritic cells, macrophages, monocytes Critical for TH1 differentiation Induces proliferation of TH1 cells, CD8 T cells, γδ T cells, and NK cells and their production of IFN-γ Enhances NK and CD8 T-cell cytotoxicity IL-13 Mast cells, TH2 cells Inhibits activation and cytokine secretion by macrophages Coactivates B-cell proliferation Upregulates class II MHC molecules and CD23 on B cells and monocytes Induces switch to IgG1 and IgE Induces VCAM-1 on endothelium IL-15 B cells, dendritic cells, macrophages, monocytes, NK cells, T cells Induces proliferation of T, NK, and activated B cells Induces cytokine production and cytotoxicity of NK cells and CD8 T cells Is chemotactic for T cells Stimulates growth of intestinal epithelium IL-17A (IL-17) IL-17F TH17 cells, γδ T cells, NKT cells, macrophages Are proinflammatory Stimulate production of cytokines (eg, TNF, IL-1β, IL-6, IL-8, G-CSF) IL-18 Monocytes, macrophages, dendritic cells Induces IFN-γ production by T cells Enhances NK-cell cytotoxicity IL-21 NKT cells, TH cells Stimulates B-cell proliferation after CD40 cross-linking Costimulates T cells Stimulates proliferation of bone marrow precursor cells IL-22 NK cells, TH17 cells, γδ cells Is proinflammatory Induces synthesis of acute phase reactants IL-23 Dendritic cells, macrophages Induces proliferation of TH cells IL-24 B cells, macrophages, monocytes, T cells Suppresses tumor cell growth Induces apoptosis in tumor cells IL-27 Dendritic cells, monocytes macrophages Induces TH1 cells IL-32 NK cells, T cells Is proinflammatory Participates in activation-induced T cell apoptosis IL-33 Stromal cells, dendritic cells Induces TH2 cytokines. Promotes chemotaxis of basophils and mast cells Interferons (IFN) IFN-α Leukocytes Inhibits viral replication Increases class I MHC expression IFN-β Fibroblasts Inhibits viral replication Increases class I MHC expression IFN-γ NK cells, TC1 cells, TH1 cells Inhibits viral replication Increases class I and II MHC expression Activates macrophages Antagonizes several actions of IL-4 Inhibits proliferation of TH2 cells Tumor necrosis factors (TNF) TNF-α (cachectin) B cells, dendritic cells, macrophages, mast cells, monocytes, NK cells, TH cells Is cytotoxic to tumor cells Causes cachexia Induces secretion of several cytokines (eg, IL-1, GM-CSF, IFN-γ) Induces E-selectin on endothelium Activates macrophages Is antiviral TNF-β (lymphotoxin) TC cells, TH1 cells Is cytotoxic to tumor cells and antiviral Enhances phagocytosis by neutrophils and macrophages Is involved in lymphoid organ development Colony-stimulating factors (CSF) G-CSF Endothelial cells, fibroblasts Stimulates growth of neutrophil precursors GM-CSF Endothelial cells, fibroblasts, macrophages, mast cells, TH cells Stimulates growth of precursors of monocytes, neutrophils, eosinophils, and basophils Activates macrophages M-CSF Endothelial cells, epithelial cells, fibroblasts Stimulates growth of monocyte precursors SCF Bone marrow stromal cells Stimulates stem cell division Transforming growth factors (TGF) TGF-β B cells, macrophages, mast cells, TH3 cells Is proinflammatory (eg, by chemoattraction of monocytes and macrophages) but also anti-inflammatory (eg, by inhibiting lymphocyte proliferation) Induces switch to IgA Promotes tissue repair CD = cluster of differentiation; G-CSF = granulocyte colony-stimulating factor; GM-CSF = granulocyte-macrophage colony-stimulating factor; ICAM-1 = intercellular adhesion molecule 1; MHC = major histocompatibility complex; NK = natural killer; SCF = stem cell factor; TC cell = cytotoxic T cell; TH cell = helper T cell; TNF = tumor necrosis factor; VCAM-1 = vascular cell adhesion molecule 1. Selected Cytokines Cytokine Major Sources Main Effects Interleukins (IL) IL-1α IL-1β B cells, dendritic cells, endothelium, macrophages, monocytes, NK cells Costimulates T-cell activation by enhancing production of cytokines (eg, IL-2 and its receptor) Enhances B-cell proliferation and maturation Enhances NK-cell cytotoxicity Induces IL-1, IL-6, IL-8, TNF, GM-CSF, and prostaglandin E2production by macrophages Is proinflammatory by inducing chemokines, ICAM-1, and VCAM-1 on endothelium Induces sleep, anorexia, release of tissue factor, acute phase reactants, and bone resorption by osteoclasts Is an endogenous pyrogen IL-2 TH1 cells Induces proliferation of activated T and B cells Enhances NK-cell cytotoxicity and killing of tumor cells and bacteria by monocytes and macrophages IL-4 Mast cells, NK cells, NKT cells, γδ T cells, TC2 cells, TH2 cells Induces TH2 cells Stimulates proliferation of activated B, T, and mast cells Upregulates class II MHC molecules on B cells and on macrophages and CD23 on B cells Downregulates IL-12 production and thereby inhibits TH1 cell-differentiation Increases macrophage phagocytosis Induces switch to IgG1 and IgE IL-5 Mast cells, TH2 cells Induces proliferation of eosinophils and activated B cells Induces switch to IgA IL-6 Dendritic cells, fibroblasts, macrophages, monocytes, TH2 cells Induces differentiation of B cells into plasma cells and differentiation of myeloid stem cells Induces acute phase reactants Enhances T-cell proliferation Induces TC-cell differentiation Is a pyrogen IL-7 Bone marrow and thymus stromal cells Induces differentiation of lymphoid stem cells into T- and B-cell precursors Activates mature T cells IL-8 (chemokine) Endothelial cells, macrophages, monocytes Mediates chemotaxis and activation of neutrophils IL-9 TH cells Induces proliferation of thymocytes Enhances mast cell growth Acts synergistically with IL-4 to induce switch to IgG1 and IgE IL-10 B cells, macrophages, monocytes, TC cells, TH2 cells, regulatory T cells Inhibits IL-2 secretion by human TH1 cells Downregulates production of class II MHC molecules and cytokines (eg, IL-12) by monocytes, macrophages, and dendritic cells and thereby inhibits TH1-cell differentiation Inhibits T-cell proliferation Enhances B-cell differentiation IL-12 B cells, dendritic cells, macrophages, monocytes Critical for TH1 differentiation Induces proliferation of TH1 cells, CD8 T cells, γδ T cells, and NK cells and their production of IFN-γ Enhances NK and CD8 T-cell cytotoxicity IL-13 Mast cells, TH2 cells Inhibits activation and cytokine secretion by macrophages Coactivates B-cell proliferation Upregulates class II MHC molecules and CD23 on B cells and monocytes Induces switch to IgG1 and IgE Induces VCAM-1 on endothelium IL-15 B cells, dendritic cells, macrophages, monocytes, NK cells, T cells Induces proliferation of T, NK, and activated B cells Induces cytokine production and cytotoxicity of NK cells and CD8 T cells Is chemotactic for T cells Stimulates growth of intestinal epithelium IL-17A (IL-17) IL-17F TH17 cells, γδ T cells, NKT cells, macrophages Are proinflammatory Stimulate production of cytokines (eg, TNF, IL-1β, IL-6, IL-8, G-CSF) IL-18 Monocytes, macrophages, dendritic cells Induces IFN-γ production by T cells Enhances NK-cell cytotoxicity IL-21 NKT cells, TH cells Stimulates B-cell proliferation after CD40 cross-linking Costimulates T cells Stimulates proliferation of bone marrow precursor cells IL-22 NK cells, TH17 cells, γδ cells Is proinflammatory Induces synthesis of acute phase reactants IL-23 Dendritic cells, macrophages Induces proliferation of TH cells IL-24 B cells, macrophages, monocytes, T cells Suppresses tumor cell growth Induces apoptosis in tumor cells IL-27 Dendritic cells, monocytes macrophages Induces TH1 cells IL-32 NK cells, T cells Is proinflammatory Participates in activation-induced T cell apoptosis IL-33 Stromal cells, dendritic cells Induces TH2 cytokines. Promotes chemotaxis of basophils and mast cells Interferons (IFN) IFN-α Leukocytes Inhibits viral replication Increases class I MHC expression IFN-β Fibroblasts Inhibits viral replication Increases class I MHC expression IFN-γ NK cells, TC1 cells, TH1 cells Inhibits viral replication Increases class I and II MHC expression Activates macrophages Antagonizes several actions of IL-4 Inhibits proliferation of TH2 cells Tumor necrosis factors (TNF) TNF-α (cachectin) B cells, dendritic cells, macrophages, mast cells, monocytes, NK cells, TH cells Is cytotoxic to tumor cells Causes cachexia Induces secretion of several cytokines (eg, IL-1, GM-CSF, IFN-γ) Induces E-selectin on endothelium Activates macrophages Is antiviral TNF-β (lymphotoxin) TC cells, TH1 cells Is cytotoxic to tumor cells and antiviral Enhances phagocytosis by neutrophils and macrophages Is involved in lymphoid organ development Colony-stimulating factors (CSF) G-CSF Endothelial cells, fibroblasts Stimulates growth of neutrophil precursors GM-CSF Endothelial cells, fibroblasts, macrophages, mast cells, TH cells Stimulates growth of precursors of monocytes, neutrophils, eosinophils, and basophils Activates macrophages M-CSF Endothelial cells, epithelial cells, fibroblasts Stimulates growth of monocyte precursors SCF Bone marrow stromal cells Stimulates stem cell division Transforming growth factors (TGF) TGF-β B cells, macrophages, mast cells, TH3 cells Is proinflammatory (eg, by chemoattraction of monocytes and macrophages) but also anti-inflammatory (eg, by inhibiting lymphocyte proliferation) Induces switch to IgA Promotes tissue repair CD = cluster of differentiation; G-CSF = granulocyte colony-stimulating factor; GM-CSF = granulocyte-macrophage colony-stimulating factor; ICAM-1 = intercellular adhesion molecule 1; MHC = major histocompatibility complex; NK = natural killer; SCF = stem cell factor; TC cell = cytotoxic T cell; TH cell = helper T cell; TNF = tumor necrosis factor; VCAM-1 = vascular cell adhesion molecule 1.

Cytokines deliver their signals via cell surface receptors. For example, the IL-2 receptor consists of 3 chains: α, β, and γ. The receptor's affinity for IL-2 is high if all 3 chains are expressed, intermediate if only the β and γ chains are expressed, or low if only the α chain is expressed. Mutations or deletion of the γ chain is the basis for X-linked severe combined immunodeficiency (see Immunodeficiency Disorders: Severe Combined Immunodeficiency (SCID)).

Chemokines induce chemotaxis and migration of leukocytes. There are 4 subsets, defined by the number of intervening amino acids between the first 2 cysteine residues in the molecule. Chemokine receptors (CCR5 on memory T cells, monocytes/macrophages, and dendritic cells; CXCR4 on resting T cells) act as coreceptors for entry of HIV into cells.

Last full review/revision November 2012 by Peter J. Delves, PhD