The basis of all immunological activity is interaction between members of different populations which constitute the lymphoid system. When one cell touches another, it responds according to its own state and its interpretation of the identity and state of the other. The existence of the major histocompatibility complex (MHC) was revealed when the difficulty of transplanting tissues between genetically different individuals was explored. It was natural to conclude that the primary function of the system was to enable the body to recognise foreign cells and to keep them out. Thoughts then turned towards the recognition of different cells arising within the body. There are two needs: to preserve normal cells, and to destroy aberrant, potentially dangerous cells. When cells are infected by a virus they can be destroyed, together with the virus, by cytotoxic T cells only if the T cells and the target cells express the same MHC class I product on their surface membranes. Whether immunological surveillance is effective against potentially malignant mutants is still not clear. The MHC is essential for recognition of one cell of the immune system by another, for recognition of genetically foreign cells and for regulation of immunological activity.