Any molecule ranging from simple chemical compounds to complex macromolecules, which are capable of being recognised by one or more constituents of the innate and adaptive immune system, is called an ‘antigen’. In the innate immune system, antigens are the pathogen‐associated molecular patterns recognised by the pattern recognition receptors expressed on macrophages, dendritic cells and NK cells. NK cells express an array of additional sets of receptors that recognise unconventional antigens. In the adaptive immune system, the B‐lymphocyte‐expressed immunoglobulin and T‐lymphocyte‐expressed T cell receptor recognise either specific conformation on the antigen or the amino acid sequence in the peptide, respectively. The antigens, which induce tolerogenic response or an allergic response, are called tolerogens or allergens, respectively. However, all antigens do not necessarily elicit antigen‐specific immune responses; those eliciting an immune response are termed immunogens. So, antigenicity of a molecule refers to its capacity to be recognised by the immune receptors, whereas immunogenicity is its ability to induce an immune response. Thus, all immunogens are antigens but all antigens are not immunogens.

Key Concepts

  • Antigens are those molecules that are specifically recognised by the receptors of the innate and adaptive immune systems.
  • Immunogens are those molecules that are able to elicit immune responses with negative or positive effects such that immune responses are suppressed or activated, respectively. The immunogens that lead to suppression or tolerance to the immune system are called tolerogens. The immunogens that cause allergic immune response are called allergens.
  • Haptens are small molecules that are able to bind antibodies but are unable to evoke an antibody response. So, all immunogens are antigens but all antigens are not immunogens.
  • Toll‐like receptors – 12 in mouse but 10 in human – in the innate immune system recognise many patterns on antigens. Two different toll‐like receptors can combine to increase the coverage of pathogen‐associated molecular patterns recognition.
  • Natural killer (NK) cells are innate immune cells which express an array of unique germ‐line‐encoded nonrearranging receptors that recognise unconventional antigens.
  • Recognition with finer antigenic specificity is executed by immunoglobulin and T‐lymphocyte receptors in the adaptive immune system.
  • The ability to rearrange the genes of the receptors (B‐ and T‐cell receptor) in the adaptive immune system creates a huge repertoire of antigen specificity, whereas such a feature is not available with the receptors of the innate immune system.
  • Recognition of specific antigen links the recall response to antigen‐specific memory, which is a characteristic of the adaptive, but not the innate, immune system.
  • B‐cell receptors recognise virtually any antigens in a conformation‐dependent manner and do not need any processing of the antigen for its recognition, whereas the T‐cell receptor recognition of an antigen depends primarily on the sequence of amino acids in the peptide.
  • T‐cell receptors recognise primarily peptide antigens in association with major histocompatibility complex molecules that necessitate processing of the antigens before the recognition. NK T cells recognise glycolipid antigens in the context of CD1d, an MHC‐like nonpolymorphic molecule on antigen‐presenting cells.

Keywords: adaptive immunity; antigenicity; epitopes; immune recognition; antigen receptors; pattern recognition receptors; pathogen‐associated molecular patterns

Figure 1. Antigen classification. (a) All antigens do not elicit immune response. Those which induce tolerance are called tolerogens. Small molecules which cannot elicit immune response on their own are termed haptens. These haptens can evoke immune response when conjugated to a carrier protein (b). The antigens which can trigger immune response are called immunogens. Among the immunogens, some molecules can result in allergic immune response following sensitisation. These molecules are called allergens. So, all immunogens are antigens but all antigens are not necessarily immunogens.
Figure 2. Antibody binding to a protein antigen. The figure shows a cartoon of a folded protein with differently shaded residues, as numbered. Antibodies can bind the epitope formed by the residues 21, 135 and 147 or the epitope formed by 87, 88, 89 and 90. The former epitope is called noncontiguous epitope, whereas the latter is called a contiguous epitope.
Figure 3. T cells recognise a peptide through TCR–peptide–MHC ternary complex. A peptide antigen (P) is selected and complexed to an MHC molecule forming a peptide–MHC complex during antigen processing. The peptide bound by α1 and β1 domains of MHC‐II is recognised by a combination of α and β chains of TCR. The MHC‐II also binds the TCR.


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Saha, Bhaskar(Sep 2015) Antigens. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000499.pub3]