Cell-mediated immune response to bacteria, computer artwork. In the background, an invading bacterial cell (brown) is phagocytosed (engulfed) by a macrophage, a type of white blood cell. Fragments of protein (antigen) from the bacterial cell are then displayed on the macrophage's surface (foreground). The antigen can be recognised by helper T lymphocytes, which secrete chemicals such as cytokines to activate other immune cells, including macrophages and cytotoxic T lymphocytes, which eliminate the pathogen.