The immune system has for function to maintain the health of an organism and to protect it from infectious diseases (infections by bacteria, virus, parasites) or from cancers (due to the proliferation of tumour cells). The immune system is a complex of organs (thymus, bone marrow, spleen, lymph nodes disseminated in the body), and of highly specialized cells (B cells, T cells, etc.) which act as sentries on the lookout for the foreign antigens.
B cells produce antibodies or immunoglobulins (IG) that mark foreign antigens (bacteria...) for their elimination or their destruction by other cells of the immune system.
There are two populations of T cells: the helper and the cytotoxic T cells. The first help the immune responses whereas the second attack and destroy cells infected by a virus or tumour cells. The helper and cytotoxic T cells are activated and functional after having recognized the foreign antigens. This recognition is performed by their T cell receptors (TR) which bind specifically to an antigen fragment (peptide derived from the foreign protein) presented on the surface of the cells by the proteins belonging to the major histocompatibility (MH) human leucocyte antigens, HLA in human).
An epitope is the part of the antigen which is specifically recognized by an antibody (B cell epitope) or a T cell receptor (T cell epitope).