T cells (T 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-presenting cell (such as a macrophage or dendritic cell).

The antigen-presenting 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-presenting cell where it is recognized by receptors on the T cell.