An antigen is defined as a substance that reacts with antibody molecules and antigen receptors on lymphocytes.With the adaptive immune responses, the body is able to recognize any conceivable antigen it may eventually encounter. Among this large variety of B-cell receptors and T-cell receptors there is bound to be at least one that has an epitope-binding site able to fit, at least to some degree, any antigen the immune system eventually encounters. In order to recognize this immense number of different epitopes, the body produces 10 7 or more distinct clones of both B-lymphocytes and T-lymphocytes, each with a unique B-cell receptor or T-cell receptor. It is estimated that the human body has the ability to recognize 10 7or more different epitopes and make up to 10 9 different antibodies, each with a unique specificity. The TCR of CD4 -CD8 - T-lymphocytes and non-MHC restricted CD4 + and CD8 + lymphocytes can recognize epitopes of lipid or glycolipid antigens after they have been attached to CD1 molecules on antigen-presenting cells or in some cases, epitopes directly on antigens. The terminal portion of the variable domains provides specificity for binding peptides of protein antigens after the protein has been unfolded, broken into peptides, and bound to a MHC molecule, while the terminus of the constant region becomes anchored to the cytoplasmic membrane of the T-lymphocyte.
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