Cell mediated immunity involves two separate populations of T cells: helper T cells (THs) and cytotoxic T cells (TCs). Viruses and some bacteria replicate in the cytoplasm of infected cells where they are inaccessible to antibodies. Cytotoxic T cells kill these infected cells either by inducing them to undergo programmed cell death (apoptosis) or by necrosis. Cytotoxic T cells also play a role in graft rejection and tumour immunity.
Apoptosis results in DNA fragmentation and destruction of the cell from within. Apoptotic cells are rapidly ingested and digested by phagocytes, allowing efficient removal of cells without an inflammatoryvresponse. Cell death by necrosis results in inflammation. These mechanisms prevent the release of infectious viruses to infect other healthy cells.
