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Overview of the Immune System


Peter J. Delves

, PhD, University College London, London, UK

Last full review/revision Apr 2020| Content last modified Apr 2020
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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 abnormal cells that derive from host tissues. Any molecule capable of being recognized by the immune system is considered an antigen (Ag).

The skin, cornea, and mucosa of the respiratory, gastrointestinal, and genitourinary tracts form a physical barrier that is the body's first line of defense. Some of these barriers also have active immune functions:

  • Outer, keratinized epidermis: Keratinocytes in the skin secrete antimicrobial peptides (defensins), and sebaceous and sweat glands secrete microbe-inhibiting substances (eg, lactic acid, fatty acids). Also, many immune cells (eg, mast cells, intraepithelial lymphocytes, antigen-sampling Langerhans cells) reside in the skin.

  • Cornea: Neutrophils reach the cornea through vessels at the limbus and kill microbes by phagocytosis.

  • Mucosa of the respiratory, gastrointestinal, and genitourinary tracts: The mucus contains antimicrobial substances, such as lysozyme, lactoferrin, and secretory immunoglobulin (Ig) A antibody (SIgA).

Breaching of anatomic barriers can trigger 2 types of immune response:

  • Innate

  • Acquired

Many molecular components (eg, complement, cytokines, acute phase reactants) participate in both innate and acquired immunity.

Innate immunity

Innate (natural) immunity does not require prior exposure to an antigen (ie, immunologic memory) to be effective. Thus, it can respond immediately to an invader. Innate immunity recognizes mainly antigen molecules that are broadly distributed rather than specific to one organism or cell.

Components include

  • Phagocytic cells (eg, neutrophils, monocytes, macrophages)

  • Innate lymphoid cells (eg, natural killer [NK] cells)

  • Polymorphonuclear leukocytes

Phagocytic cells (neutrophils in blood and tissues, monocytes in blood, macrophages in tissues) ingest and destroy invading antigens. Attack by phagocytic cells can be facilitated when antigens are coated with antibody (Ab), which is produced as part of acquired immunity, or when complement proteins opsonize antigens.

Natural killer cells kill virus-infected cells and some tumor cells.

Polymorphonuclear leukocytes (neutrophils, eosinophils, basophils) and mononuclear cells (monocytes, macrophages, mast cells) release inflammatory mediators.

Acquired immunity

Acquired (adaptive) immunity requires prior exposure to an antigen and thus takes time to develop after the initial encounter with a new invader. Thereafter, response is quick. The system remembers past exposures and is antigen-specific.

Components include

  • T cells

  • B cells

Acquired immunity includes

  • Cell-mediated immunity: Derived from certain T-cell responses

  • Humoral immunity: Derived from B-cell responses (B cells secrete soluble antigen-specific antibody)

B cells and T cells work together to destroy invaders. Tissue-based antigen-presenting cells are needed to present antigens to T cells.

Immune Response

Successful immune defense requires activation, regulation, and resolution of the immune response.


The cells of the immune system are activated when a foreign antigen (Ag) is recognized by cell surface receptors. These cell surface receptors may be

  • Broadly specific (eg, pattern-recognition receptors such as Toll-like, mannose, and scavenger receptors on dendritic and other cells)

  • Highly specific (antibodies expressed on B cells or T-cell receptors expressed on T cells)

Broadly specific receptors recognize common microbial pathogen-associated molecular patterns such as gram-negative lipopolysaccharide, gram-positive peptidoglycans, bacterial flagellin, unmethylated cytosine-guanosine dinucleotides (CpG motifs), and viral double-stranded RNA. These receptors can also recognize molecules that are produced by stressed or infected human cells (called damage-associated molecular patterns).

Activation may also occur when antibody-antigen and complement-microorganism complexes bind to surface receptors for the crystallizable fragment (Fc) region of IgG (Fc-gamma R) and for C3b and iC3b.

Once recognized, an antigen, antigen-antibody complex, or complement-microorganism complex is internalized. Most microorganisms are killed after they are phagocytosed, but others inhibit the phagocyte’s intracellular killing ability (eg, mycobacteria that have been engulfed by a macrophage inhibit that cell's killing ability). In such cases, T cell–derived cytokines, particularly interferon-gamma (IFN-gamma), stimulate the phagocyte to produce more lytic enzymes and other microbicidal products and thus enhance its ability to kill or sequester the microorganism.

Unless antigen is rapidly phagocytosed and entirely degraded (an uncommon event), the acquired immune response is recruited via recognition of antigen by the highly specific receptors on the surface of B and T cells. This response begins in

  • The spleen for circulating antigen

  • Regional lymph nodes for tissue antigen

  • Mucosa-associated lymphoid tissues (eg, tonsils, adenoids, Peyer patches) for mucosal antigen

For example, Langerhans dendritic cells in the skin phagocytose antigen and migrate to local lymph nodes; there, peptides derived from the antigen are expressed on the surface of dendritic cells within class II major histocompatibility complex (MHC) molecules, which present the peptide to CD4 helper T (Th) cells. When the Th cell engages the MHC-peptide complex and receives various costimulatory signals (which can be inhibited by some immunosuppressive drugs), it is activated to express receptors for the cytokine interleukin (IL)-2 and secretes several cytokines. Each subset of Th cells secretes different combinations of substances and thus effects different immune responses.

Class II MHC molecules typically present peptides derived from extracellular (exogenous) antigen (eg, from many bacteria) to CD4 Th cells; in contrast, class I MHC molecules typically present peptides derived from intracellular (endogenous) antigens (eg, from viruses) to CD8 cytotoxic T cells. The activated cytotoxic T cell then kills the infected cell.


The immune response must be regulated to prevent overwhelming damage to the host (eg, anaphylaxis, widespread tissue destruction). Regulatory T cells (most of which express Foxp3 transcription factor) help control the immune response via secretion of immunosuppressive cytokines, such as IL-10 and transforming growth factor-beta (TGF-beta), or via cell contact dependent mechanisms.

These regulatory cells help prevent autoimmune responses and probably help resolve ongoing responses to nonself antigen.


The immune response resolves when antigen is sequestered or eliminated from the body. Without stimulation by antigen, cytokine secretion ceases, and activated cytotoxic T cells undergo apoptosis. Apoptosis tags a cell for immediate phagocytosis, which prevents spillage of the cellular contents and development of subsequent inflammation. T and B cells that have differentiated into memory cells are spared this fate.

Geriatrics Essentials

With aging, the immune system becomes less effective in the following ways:

  • The immune system becomes less able to distinguish self from nonself, making autoimmune disorders more common.

  • Macrophages destroy bacteria, cancer cells, and other antigens more slowly, possibly contributing to the increased incidence of cancer among older adults.

  • T cells respond less quickly to antigens.

  • There are fewer lymphocytes that can respond to new antigens.

  • The aging body produces less complement in response to bacterial infections.

  • Although overall antibody concentration does not decline significantly, the binding affinity of antibody to antigen is decreased, possibly contributing to the increased incidence of pneumonia, influenza, infective endocarditis, and tetanus and the increased risk of death due to these disorders among older adults. These changes may also partly explain why vaccines are less effective in older adults.

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