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		Overview of the Immune System

T cells (lymphocytes), as part of the immune surveillance system, must be able to recognize substances that do not belong to the body (foreign antigens). However, they cannot directly recognize an antigen. They need the help of an antigen-processing cell (such as a macrophage). The antigen-processing cell engulfs the antigen. Then enzymes in the cell break the antigen into fragments, which are combined with the cell's identification molecules (called major histocompatibility complex molecules, or human leukocyte antigens [HLAs]). The combined HLA and antigen fragment moves to the surface of the antigen-processing cell where it is recognized by receptors on the T cell.