T cells are so called because they are predominantly produced in the thymus. They recognise foreign particles (antigen) by a surface expressed, highly variable, T cell receptor (TCR). There are two major types of T cells: the helper T cell and the cytotoxic T cell. As the names suggest helper T cells ‘help’ other cells of the immune system, whilst cytotoxic T cells kill virally infected cells and tumours.
Unlike antibody, the TCR cannot bind antigen directly. Instead it needs to have broken-down peptides of the antigen ‘presented’ to it by an antigen presenting cell (APC). The molecules on the APC that present the antigen are called major histocompatibility complexes (MHC). There are two types of MHC: MHC class I and MHC class II. MHC class I presents to cytotoxic T cells; MHC class II presents to helper T cells.
