Antigen presenting cell (APC), computer artwork. Cell processes of an APC (right) present a bacterial fragment, or antigen (orange), to a T lymphocyte (left). APCs include macrophages, dendritic cells and monocytes. They play a role in the body's immune system by recognising and phagocytosing (engulfing and destroying) pathogens. They then present the digested parts (antigens) of the phagocytosed pathogens to other immune system cells. The antigen is recognised by helper T lymphocytes, which secrete chemicals such as cytokines to activate other immune cells against the pathogen, including T cytotoxic cells and antibody-producing B cells.