Antigen
In immunology, antigens (Ag) are structures (aka substances) specifically bound by antibodies (Ab) or a cell surface version of Ab ~ B cell antigen receptor (BCR). The term antigen originally described a structural molecule that binds specifically to an antibody only in the form of native antigen. It was expanded later to refer to any molecule or a linear molecular fragment after processing the native antigen that can be recognized by T-cell receptor (TCR). BCR and TCR are both highly variable antigen receptors diversified by somatic V(D)J recombination. Both T cells and B cells are cellular components of adaptive immunity. The Ag abbreviation stands for an antibody generator.
Antigen presentation
APCs usually internalise exogenous antigens by endocytosis, but also by pinocytosis, macroautophagy, endosomal microautophagy or chaperone-mediated autophagy. In the first case, after internalisation, the antigens are enclosed in vesicles called endosomes. There are three compartements involved in this antigen presentation pathway: early endosomes, late endosomes or endolysosomes and lysosomes, where antigens are hydrolized by lysosome-associated enzymes (acid-dependent hydrolases, glycosidases, proteases, lipases). This process is favored by gradual reduction of the pH. The main proteases in endosomes are cathepsins and the result is the degradation of the antigens into oligopeptides.