Antigens

Foreign substances that induce specific immune responses or substances that are targets of such response are called antigens. The definition of an antigen responds also to the requirement that this is a substance that generates the production of antibodies. Number of infectious antigens, like Corona, Ebola, West Nile or avian influenza H5N1 viruses as well as the improvement of our knowledge of autoantigens, rise new interest into immune complexes since the major way one gets rid of those viruses is through humoral, antibody-mediated defense hence antibody binding. Synthesis of those antibodies is not a constant, carved-in-stone event, rather antigen-recognising formulas of our body like T cell receptor and histocompatibility complex impinge on how B lymphocytes synthesize their antibody clones. Decision as to what transforms antigens into autoantigens is subtle as reflected in the low density lipoproteins, which, once oxidized (OxLDL), become autoantigens and induce immune complex disease (J Lipid Res 2006;47(9):1975).

Epitope spreading draws our attention to the possibility that the initial immune response might become broadened by immunization with a single-peptide antigen or a multi-determinant antigen upon repeat exposure.

Another sort of epitope spreading is antigenic drift. Vaccination with new but cross-reactive antigenic variant of a virus can induce antibody formation directed at epitopes characterizing influenza viruses encountered during earlier epidemics ('antigenic sin' ) . Original antigenic sin (OAS) is rare in healthy individuals but higher affinity antibody against an influenza virus strain used for booster vaccination in an individual who previously encountered the virus can be observed (Wrammert J et al, Nature 2008; 453(7195):667-71

The yellow triangle, symbol of the antigen or a vaccine, is presented by the major histocompatibility complex (MHC) to the immune system represented here by a T lymphocyte.
TCR: T cell receptor, the recognition unit on a T lymphocyte which looks at the antigen and welcomes it or ignores it.

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