An antigen is any molecule that stimulates an immune response. Most antigens are proteins or polysaccharides, though small molecules coupled to carrier proteins (haptens) can also be antigenic. The segment of an antigenic molecule to which its cognate antibody binds is termed an epitope or antigenic determinant.

allergen : allergic reactions : antigenic determinant : autoantigenic : autoimmune disorders : class I histocompatibility molecule (MHC I) : class II histocompatibility molecule (MHC II) : immunogen : endogenous : epitope : exogenous : lipid Ag : pathogen-associated molecular pattern (PAMP) : pattern-recognition receptor (PRR) : polysaccharide Ag : T-dependent : T-independent : tolerogen : Toll-like receptor (TLR) : tumor antigens : tumor-associated antigen (TAA) : tumor-specific antigen (TSA) ▼

Antigens are classified by immune activity as immunogens, tolerogens, or allergens according to whether the molecule in question activates the immune response, is tolerated by the immune system, or elicits an allergic response, respectively. Allergic reactions are exaggerated immune responses to molecules (allergens) that would otherwise not prove harmful. Antigens may also be classified according to their source as exogenous, endogenous, autoantigenic, or tumor antigens.

Exogenous antigens are foreign molecules that are ingested (endo-, phagocytosis) by antigen presenting cells on which the fragmented and extruded antigens are then carried on class II histocompatibility molecules (MHC II) for presentation to CD4+ Th cells. Pathogen-associated molecular patterns (PAMPs ) are small molecular sequences consistently found on pathogens that are recognized by Toll-like receptors (TLRs) and other pattern-recognition receptors (PRRs). Pattern recognition receptors (PRR) are a class of innate immune response-expressed protein receptors that respond to PAMPs.

Endogenous antigens are internally generated molecules that become presented on the cell surface in the complex with class I histocompatibility molecules (MHC I). Endogenous antigens may result from exogeneous viral or bacterial infections that have altered the host cell.

In autoimmune disorders, endogenous, self-molecules induce autimmune attack by CD8+ Tc/CTLs that have escaped negative selection in the thymus.

Tumor-specific antigens (TSAs) typically result from a tumor specific mutation and are targetted for non-self attack when displayed on class I histocompatibility molecules. Tumor-associated antigens (TAAs) are more common than TSAs, and are presented both by tumor cells and by normal cells. Tumor antigens may elicit targetting by CTLs before the tumor cells can successfully proliferate and metastasize. Unfortunately, tumors employ a variety of mechanisms to evade the immune system.

Protein antigens are T dependent in that they require T cell co-operation to induce antibody responses in B cells. Non-protein antigens, such as polysaccharides and lipids can elicit T-independent antibody responses. Such T-independent antigens are typically polymeric, so it is believed that they are able to cross-link BCR-surface-Ig sufficiently strongly to activate B cells without T cell costimulation. These T-independent polymeric antigens elicit IgM antibodies and do not demonstrate affinity maturation. However, a subclass of T cells are specialized to present lipid and glycolipid antigens – γδ T cells recognize foreign nonpeptide antigens presented by CD1 proteins, which are MHC-like-molecules specialized for the presentation of lipids.

ф activation ф anergy ф antibodies ф antigen presenting cells (APCs) ф autoimmunity ф basophils ¤ cancercell-cycle control ф class-switch recombination ф clonal selection ф dendritic cells o-o endogenous vs exogenous ф eosinophils ф granulocytes ~ growth factors ф immune cytokines ф immune response ф immune tolerance ф inflammatory response ф interferons ф isotype switching ф leukocytes ф lymphocytes ф lymphokines ф lymphoid system ф macrophages ф MHC ф monocytes ф pathogens ф pattern-recognition receptors ф phagocyte ф plasma cells ¤ proliferation ф receptorsregulation of gene expression ф secondary antibody diversification ф signaling ф surface receptors ф vaccines

Tables  Fc receptors  Immune Cytokines  Immunoglobulins  Cell Adhesion Molecules  Cell signaling  Receptor Tyrosine Kinases (RTKs)  Receptor Signal Transduction  Second Messengers 



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