Each antibody is specifically selected after binding to a certain antigen because of random somatic diversification in the antibody complementarity determining regions. A common analogy used to describe this is the fit between a lock and a key. An antigen (Ag) is any structural substance that serves as a target for the receptors of an adaptive immune response, TCR or BCR or its secreted form antibody. The antigen may originate from within the body ("self-antigen") or from the external environment ("non-self"). The immune system usually does not react to self-antigens under normal homeostatic conditions due to negative selection of T cells in the thymus and is supposed to identify and attack only "non-self" invaders from the outside world or modified/harmful substances present in the body under distressed conditions. Antigen presenting cells present antigens in the form of peptides on histocompatibility molecules. The T cells of the adaptive immune system recognize the antigens.

Depending on the antigen and the type of the histocompatibility molecule, different types of T cells activate. For T-Cell Receptor (TCR) recognition, the peptide must be processed into small fragments inside the cell and presented by a major histocompatibility complex (MHC).

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