How T Lymphocytes Recognize Antigens

An antigen is a substance that is foreign to the body. A T lymphocyte, part of the immune surveillance system, cannot directly recognize an antigen. Thus, the antigen is first engulfed by an antigen-processing cell (macrophage). Enzymes in the macrophage break the antigen into fragments, which join with major histocompatibility complex molecules. This new antigen formation moves to the surface of the macrophage, is recognized by the T-lymphocyte receptor, and binds with the T lymphocyte.