Immune System Response to Antigens

• The immune system distinguishes between two groups of foreign substances: antigens that are freely circulating in the body and self cells that display aberrant MHC proteins. • Antigens that are freely circulating in the body include molecules, viruses, and foreign cells. • Aberrant MHC proteins can originate from exogenous antigens that have been engulfed and broken down or from endogenous antigens synthesized by virus-infected and tumor cells. • The humoral response, also known as the antibody-mediated response, involves B cells that recognize antigens or pathogens circulating in the lymph or blood. • B cells recognize antigens through binding, and interleukins or helper T cells costimulate B cells, leading to B cell proliferation and production of plasma cells. • Plasma cells produce antibodies with identical antigen specificity as the antigen receptors of the activated B cells, which are then released and circulate through the body, binding to antigens. • B cells also produce memory cells, providing future immunity. • The cell-mediated response involves mostly T cells and responds to cells that display aberrant MHC markers, including cells invaded by pathogens, tumor cells, or transplanted cells. • T cells recognize self cells or antigen-presenting cells (APCs) displaying foreign antigens through binding, and interleukins costimulate activation of T cells. • If MHC-I and endogenous antigens are displayed on the plasma membrane, T cells proliferate, producing cytotoxic T cells that destroy cells displaying the antigens. • If MHC-II and exogenous antigens are displayed on the plasma membrane, T cells proliferate, producing helper T cells that release interleukins and stimulate B cells to produce antibodies and nonspecific agents (NK and macrophages) to destroy the antigens.