The immune system consists of cellular components and molecular components Molecular Components of the Immune System The immune system consists of cellular components and molecular components that work together to destroy antigens (Ags). (See also Overview of the Immune System.) Acute phase reactants are plasma... read more that work together to destroy antigens. (See also Overview of the Immune System Overview of the Immune System The immune system distinguishes self from nonself and eliminates potentially harmful nonself molecules and cells from the body. The immune system also has the capacity to recognize and destroy... read more .)
Although some antigens (Ags) can stimulate the immune response directly, T cell–dependent acquired immune responses typically require antigen-presenting cells (APCs) to present antigen-derived peptides within major histocompatibility complex (MHC) molecules.
Intracellular antigens (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 Cytomegalovirus (CMV) Infection Cytomegalovirus (CMV, human herpesvirus type 5) can cause infections that have a wide range of severity. A syndrome of infectious mononucleosis that lacks severe pharyngitis is common. Severe... read more ) can evade elimination.
Extracellular antigens (eg, from many bacteria) that are phagocytosed or endocytosed can be processed into peptides and complexed with surface class II MHC molecules on professional APCs, which specialize in presenting antigens to CD4 helper T (Th) cells. The following cells constitutively express class II MHC molecules and therefore act as professional APCs:
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 antigen, then traveling to local lymph nodes where they can activate T cells.
Follicular dendritic cells are a distinct lineage (a different cell type than conventional dendritic cells), do not express class II MHC molecules, and therefore do not present antigen to Th cells. They are not phagocytic; they have receptors for the crystallizable fragment (Fc) region of immunoglobulin (Ig) G 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.
Monocytes in the circulation are precursors to tissue macrophages. Monocytes migrate into tissues, where over approximately 8 hours, they develop into macrophages. This development occurs under the influence of macrophage colony-stimulating factor (M-CSF), which is secreted by various cell types (eg, endothelial cells, fibroblasts). At infection sites, activated T cells secrete cytokines (eg, interferon-gamma [IFN-gamma]) that induce production of macrophage migration inhibitory factor, preventing macrophages from leaving.
Macrophages are phagocytic cells present in tissues throughout the body. Depending upon the activation signals they receive, macrophages can alter their gene expression profiles and develop into polarized M1 or M2 subsets (see table ). M1, "classically activated," pro-inflammatory macrophages are stimulated by cytokines such as IFN-gamma and by various microbial components (eg, lipopolysaccharide). M2 "alternatively activated" anti-inflammatory macrophages are stimulated predominantly by cytokines such as IL-4 and IL-13. M1 macrophages are strongly microbicidal, promote Th1 responses, and secrete proinflammatory cytokines (eg, TNF-alpha), whereas M2 macrophages secrete immunosuppressive cytokines (eg, IL-10, transforming growth factor-beta [TGF-beta]), are important in resolving inflammation and in promoting tissue remodelling. M2 macrophages can also contribute to fibrosis by producing profibrotic factors such as TGF-beta.
B cells' primary function is to develop into plasma cells, which manufacture and secrete antibodies.
The 2 main types of lymphocytes are
B cells (which mature in bone marrow)
T cells (which mature in the thymus)
The main types of lymphocytes are morphologically indistinguishable but have different immune functions. They can be distinguished by antigen-specific surface receptors and other cell surface molecules called clusters of differentiation (CDs), whose presence or absence define some subsets. More than 300 CDs have been identified many of which are absent from lymphocytes but present on other cells of the immune system. CD molecules function in cell adhesion, cell signaling, as receptors for the Fc region of immunoglobulins, as receptors for components of the complement system, and others. (For further information on CD molecules, see the Human Cell Differentiation Molecules web site.) Each lymphocyte recognizes a specific antigen via cell surface B-cell receptors (BCRs, transmembrane antibody) or T-cell receptors (TCRs).
About 5 to 15% of lymphocytes in the blood are B cells; they are also present in the bone marrow, spleen, lymph nodes, and mucosa-associated lymphoid tissues.
B cells can present antigen to T cells and release cytokines, but their primary function is to develop into plasma cells, which manufacture and secrete antibodies Antibodies The immune system consists of cellular components and molecular components that work together to destroy antigens (Ags). (See also Overview of the Immune System.) Acute phase reactants are plasma... read more .
Patients with B-cell immunodeficiencies (eg, X-linked agammaglobulinemia X-linked Agammaglobulinemia X-linked agammaglobulinemia is characterized by low levels or absence of immunoglobulins and absence of B cells, leading to recurrent infections with encapsulated bacteria. (See also Overview... read more ) are especially susceptible to recurrent bacterial infections.
After random rearrangement of the immunoglobulin (Ig) variable, diversity, and joining genes that encode the variable region of an antibody, B cells collectively have the potential to recognize an almost limitless number of unique antigens. 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. At this point, any cells that interact with self antigen (autoimmune cells) are removed from the immature B cell population via inactivation (anergy) or apoptosis, thereby ensuring immune tolerance. Cells that are not removed (ie, those that recognize nonself antigen) continue to develop into mature naive B cells, leave the bone marrow, and enter peripheral lymphoid organs, where they may encounter antigens.
Their response to antigen has 2 stages:
Primary immune response: When mature naive B cells first encounter antigen, they become lymphoblasts, undergo clonal proliferation, and differentiate into memory cells, which can respond to the same antigen in the future, or into mature antibody-secreting plasma cells. After first exposure, there is a latent period of days before antibody is produced. Initially, only IgM is produced. After that, with the help of T cells, B cells can further rearrange their Ig genes and undergo class switching to produce antigen-specific IgG, IgA, or IgE. Thus, at first exposure, the response is slow and initially provides limited protective immunity.
Secondary (anamnestic or booster) immune response: When memory B cells and Th cells are re-exposed to the antigen, the memory B cells rapidly proliferate, differentiate into mature plasma cells, and promptly produce large amounts of antibody (chiefly IgG because of a T cell–induced class switch). The antibody also has stronger binding affinity to the antigen due to mutation in the genes encoding the antibody variable regions. The antibody is released into the blood and other tissues, where it can react with antigen. Thus, after re-exposure, the immune response is faster and more effective.
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:
In thymic selection, T cells that react to self antigen presented by self MHC molecules (or react strongly to self MHC molecules regardless of the antigen presented) are eliminated by apoptosis, limiting the likelihood of autoimmunity. Only T cells that can recognize nonself antigen complexed to self MHC molecules survive; they leave the thymus for peripheral blood and lymphoid tissues.
Most mature T cells express either CD4 or CD8 and have an antigen-binding, Ig-like, surface receptor called the T-cell receptor (TCR). There are 2 types of TCR:
Alpha-beta TCR: Composed of TCR alpha and beta chains; present on most T cells
Gamma-delta TCR: Composed of TCR gamma and delta chains; present on a minor population of T cells
Genes that encode the TCR, like Ig genes, are rearranged, resulting in defined specificity and affinity for antigen. Most T cells (those with an alpha-beta TCR) recognize antigen-derived peptide displayed along with the MHC molecule of an antigen-presenting cell. T cells with gamma-delta TCR recognize protein antigen directly or recognize lipid antigen displayed by an MHC-like molecule called CD1. The number of T-cell specificities is almost limitless.
For alpha-beta T cells to be activated, the TCR must engage with antigen-MHC (see figure ). Costimulatory accessory molecules must also interact (eg, CD28 on the T cell interacts with CD80 and CD86 on the antigen-presenting cell); otherwise, the T cell exposed to antigen becomes anergic or dies by apoptosis. Some accessory molecules (eg, CTLA-4 [cytotoxic T-lymphocyte antigen 4] on the T cell, which also interacts with CD80 and CD86 on the antigen-presenting cell, PD-1 [programmed cell death protein 1] on the T cell, which interacts with PD-L1 [programmed cell death protein ligand 1] on the antigen-presenting cell) inhibit previously activated T cells and thus dampen the immune response.
Molecules such as CTLA-4 and PD-1, and their ligands, are termed checkpoint molecules because they signal that the T cell needs to be restrained from continuing its activity. Cancer cells that express checkpoint molecules may thus be protected from the immune system by restraining the activity of tumor-specific T cells.
Monoclonal antibodies that target checkpoint molecules on either T cells or on tumor cells (termed checkpoint inhibitors, see table ) are used to prevent downregulation of antitumor immune responses and effectively treat some heretofore resistant cancers and boost the antitumor response. However, because checkpoint molecules are also involved in preventing other types of immune response (such as self-directed autoimmune reactions), checkpoint inhibitors can permit severe immune-related inflammatory and autoimmune reactions to occur (both systemic and organ specific) or exacerbate autoimmune disorders.
Polymorphisms in the CTLA-4 gene are associated with certain autoimmune disorders, including Graves disease Etiology and type 1 diabetes mellitus Type 1 diabetes Diabetes mellitus is impaired insulin secretion and variable degrees of peripheral insulin resistance leading to hyperglycemia. Early symptoms are related to hyperglycemia and include polydipsia... read more .
Two-Signal Model for T-cell Activation
The alpha (α) and beta (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 (first signal) through the accessory CD3 chains. However, without a second (coactivation) signal, the T cell is anergic or tolerant.
The TCR is structurally homologous to the B-cell receptor; the α and β (or gamma [γ] and delta [δ]) chains have constant (C) and variable (V) regions. (1) = first signal; (2) = second signal.
Helper T (Th) cells are usually CD4 but may be CD8. They differentiate from Th0 cells into one of the following:
Th1 cells: In general, Th1 cells promote cell-mediated immunity via cytotoxic T cells and macrophages and are thus particularly involved in defense against intracellular pathogens (eg, viruses). They can also promote the production of some antibody classes.
Th2 cells: Th2 cells are particularly adept at promoting antibody production by B cells (humoral immunity) and thus are particularly involved in directing responses aimed at extracellular pathogens (eg, bacteria, parasites).
Th17 cells: Th17 cells promote tissue inflammation.
Tfh cells (T follicular helper cells): Tfh cells promote B-cell responses in the germinal centers of secondary lymphoid tissues
Each cell type secretes several cytokines Cytokines The immune system consists of cellular components and molecular components that work together to destroy antigens (Ags). (See also Overview of the Immune System.) Acute phase reactants are plasma... read more (see table ). Different patterns of cytokine production identify other Th-cell functional phenotypes. Depending on the stimulating pathogen, Th1 and Th2 cells can, to a certain extent, downregulate each other's activity, leading to dominance of a Th1 or a Th2 response.
Any T cells that can recognize peptides derived from self antigens are normally eliminated during T cell development in the thymus. If such T cells are not deleted, they can potentially develop into autoimmune Th1, Th2, or Th17 cells that may drive development of autoimmune disease.
The distinction between the different Th cells is clinically relevant. For example, a Th1 response dominates in tuberculoid leprosy Symptoms and Signs , and a Th2 response dominates in lepromatous leprosy Classification of leprosy Leprosy is a chronic infection usually caused by the acid-fast bacilli Mycobacterium leprae or the closely related organism M. lepromatosis. These organisms have a unique tropism... read more . A Th1 response is characteristic of certain autoimmune disorders (eg, type 1 diabetes mellitus Diabetes Mellitus (DM) Diabetes mellitus is impaired insulin secretion and variable degrees of peripheral insulin resistance leading to hyperglycemia. Early symptoms are related to hyperglycemia and include polydipsia... read more , multiple sclerosis Multiple Sclerosis (MS) Multiple sclerosis (MS) is characterized by disseminated patches of demyelination in the brain and spinal cord. Common symptoms include visual and oculomotor abnormalities, paresthesias, weakness... read more ). A Th2 response promotes IgE production and development of allergic disorders, as well as helps B cells produce autoantibodies in some autoimmune disorders (eg, Graves disease Hyperthyroidism Hyperthyroidism is characterized by hypermetabolism and elevated serum levels of free thyroid hormones. Symptoms include palpitations, fatigue, weight loss, heat intolerance, anxiety, and tremor... read more , myasthenia gravis Myasthenia Gravis Myasthenia gravis is characterized by episodic muscle weakness and easy fatigability caused by autoantibody- and cell-mediated destruction of acetylcholine receptors. It is more common among... read more ). Th17 cells, via their role in inflammation, may also contribute to autoimmune disorders such as psoriasis Psoriasis Psoriasis is an inflammatory disease that manifests most commonly as well-circumscribed, erythematous papules and plaques covered with silvery scales. Multiple factors contribute, including... read more and rheumatoid arthritis Rheumatoid Arthritis (RA) Rheumatoid arthritis is a chronic systemic autoimmune disease that primarily involves the joints. Rheumatoid arthritis causes damage mediated by cytokines, chemokines, and metalloproteases.... read more . Patients with immunodeficiencies characterized by defective Th17 cells (eg, hyper-IgE [Job] syndrome) are especially susceptible to infection with Candida albicans Candidiasis Candidiasis is infection by Candida species (most often C. albicans), manifested by mucocutaneous lesions, fungemia, and sometimes focal infection of multiple sites. Symptoms depend... read more and Staphylococcus aureus Staphylococcal Infections Staphylococci are gram-positive aerobic organisms. Staphylococcus aureus is the most pathogenic; it typically causes skin infections and sometimes pneumonia, endocarditis, and osteomyelitis... read more .
Regulatory (suppressor) T (Treg) cells mediate suppression of immune responses and usually express the Foxp3 transcription factor. They comprise functional subsets of CD4 or CD8 T cells:
Natural Treg cells develop within the thymus
Induced Treg cells develop from conventional T cells upon encounter with antigen in the periphery
Regulatory T cells secrete cytokines with immunosuppressive properties (eg, transforming growth factor [TGF]-beta and interleukin [IL]-10) deplete immunostimulatory interleukin-2 (IL-2) by expressing high levels of the CD25 component of the IL-2 receptor, or suppress the immune response by mechanisms that require cell-to-cell contact and involve cell surface molecules (eg, CTLA-4). The main role of Treg cells is to prevent excessive immune responses by contributing to the resolution of inflammation and by curbing allergic and autoimmune activity, Patients with functional mutations in Foxp3 develop the autoimmune disorder IPEX syndrome IPEX Syndrome IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) is an X-linked recessive syndrome involving aggressive autoimmunity. This rare disorder results from mutation of the transcriptional... read more (immunodysregulation, polyendocrinopathy, enteropathy, X-linked syndrome).
Cytotoxic T (Tc) cells are usually CD8 but may be CD4. They are vital for eliminating intracellular pathogens, especially viruses. Tc cells play a role in organ transplant rejection.
Tc-cell development involves 3 phases:
A precursor cell that, when appropriately stimulated, can differentiate into a Tc 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 antigen-MHC combination
Fully activated Tc cells, like natural killer (NK) cells, can kill an infected target cell by inducing apoptosis.
Tc cells can secrete cytokines and, like Th cells, have been divided into types Tc1, Tc2, and Tc17 based on their patterns of cytokine production.
Tc cells may be
Syngeneic: Generated in response to self (autologous) cells whose MHC presents peptides derived from 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 Tc 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).
Natural killer T (NKT) cells are a distinct subset of T cells. Activated NKT cells secrete IL-4 and interferon-gamma and may help regulate immune responses. NKT cells differ from NK cells Natural Killer (NK) Cells The immune system consists of cellular components and molecular components that work together to destroy antigens. (See also Overview of the Immune System.) Although some antigens (Ags) can... read more in phenotype and certain functions.
Mast cells are present in mucosal tissues and connective tissues throughout the body and are functionally similar to basophils Basophils The immune system consists of cellular components and molecular components that work together to destroy antigens. (See also Overview of the Immune System.) Although some antigens (Ags) can... read more circulating in the blood.
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 Type I Allergic (including atopic) and other hypersensitivity disorders are inappropriate or exaggerated immune reactions to foreign antigens. Inappropriate immune reactions include those that are... read more . They have high-affinity receptors for IgE called Fc-epsilon RI (FcεRI). Degranulation can be triggered by cross-linking of IgE receptors or by the anaphylatoxin complement Complement System The complement system is an enzyme cascade that helps defend against infection via activation of a local inflammatory response. Many complement proteins occur in serum as inactive enzyme precursors... read more fragments C3a and C5a.
Natural Killer (NK) Cells
Typical natural killer (NK) cells belong to a category of cells collectively referred to as innate lymphoid cells (which also includes ILC1, ILC2, and ILC3). NK cells constitute 5 to 15% of peripheral blood mononuclear cells and have a round nucleus and granular cytoplasm. They induce apoptosis in infected or abnormal cells by a number of pathways. Like other innate lymphoid cells, they lack antigen-specific receptors; however, some NK cells have a form of 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 prevents 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.
NK cells can also secrete several cytokines Cytokines The immune system consists of cellular components and molecular components that work together to destroy antigens (Ags). (See also Overview of the Immune System.) Acute phase reactants are plasma... read more (eg, IFN-gamma, IL-1, TNF-alpha); they are a major source of IFN-gamma. By secreting IFN-gamma, NK cells can influence the acquired immune system by promoting differentiation of type 1 helper T (Th1) cells and inhibiting that of type 2 helper (Th2) cells.
Patients with NK-cell deficiencies (eg, some types of severe combined immunodeficiency Severe Combined Immunodeficiency (SCID) Severe combined immunodeficiency is characterized by low to absent T cells and a low, high, or normal number of B cells and natural killer cells. Most infants develop opportunistic infections... read more ) are especially susceptible to herpesvirus infections Overview of Herpesvirus Infections Eight types of herpesviruses infect humans (see table ). After initial infection, all herpesviruses remain latent within specific host cells and may subsequently reactivate. Clinical syndromes... read more and human papillomavirus infections Human Papillomavirus (HPV) Infection Human papillomavirus (HPV) infects epithelial cells. Most of the > 100 subtypes infect cutaneous epithelium and cause skin warts; some types infect mucosal epithelium and cause anogenital warts... read more , while an excess of NK cells may contribute to the development of autoimmune disease Autoimmune Disorders In autoimmune disorders, the immune system produces antibodies to an endogenous antigen (autoantigen). The following types of hypersensitivity reactions may be involved: Type II: Antibody-coated... read more .
Polymorphonuclear leukocytes, also called granulocytes because their cytoplasm contains granules, include
Polymorphonuclear leukocytes occur in the circulation and have multilobed nuclei.
Neutrophils constitute 40 to 70% of total circulating white blood cells; 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, use adhesion molecules on blood vessel endothelium to leave the circulation and enter tissues. Their purpose is to phagocytose and digest pathogens. Microorganisms are killed when phagocytosis generates lytic enzymes and reactive oxygen 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 fibrous structures called neutrophil extracellular traps (NETs) in the surrounding tissues; these structures facilitate killing by trapping bacteria and focusing enzyme activity.
Patients with immunodeficiencies that affect the ability of phagocytes to kill pathogens (eg, chronic granulomatous disease Chronic Granulomatous Disease (CGD) Chronic granulomatous disease is characterized by white blood cells that cannot produce activated oxygen compounds and by defects in phagocytic cell microbicidal function. Manifestations include... read more ) are especially susceptible to chronic bacterial and fungal infections.
Eosinophils constitute up to 5% of circulating white blood cells.
They target organisms too large to be engulfed. Eosinophils kill by secreting toxic substances (eg, reactive oxygen compounds similar to those produced in neutrophils), major basic protein (which is toxic to parasites), eosinophil cationic protein, and several enzymes.
Circulating eosinophils, which increase in response to invasive parasitic infections, some cancers, and as part of an allergic response, are also a major source of inflammatory mediators (eg, prostaglandins, leukotrienes, platelet-activating factor, many cytokines).
Basophils constitute < 5% of circulating white blood cells. Basophils share several characteristics with mast cells Mast Cells The immune system consists of cellular components and molecular components that work together to destroy antigens. (See also Overview of the Immune System.) Although some antigens (Ags) can... read more , although the 2 cell types have distinct lineages. Both have high-affinity receptors for IgE called Fc-epsilon RI (FcεRI). When these cells encounter certain antigens, 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).