The development of most immune responses depends on the recognition of what is self and what is not self. This determination must be clear and must be done in a very general way. This recognition is achieved by the expression of specialized surface markers on human cells. The major group of markers involved in this recognition consists of surface proteins. These are referred to as the major histocompatibility complex3 (MHC) or major histocompatibility antigens. Proteins ex pressed on the cell surfaces are class I MHCs and class II MHCs. Both classes are highly polymorphic and so are highly specific to each individual. Class I MHCs can be found on virtually all nucleated cells in the human body, whereas class II MHC molecules are associated only with B lymphocytes and macrophages. Class I MHCs are markers that are recognized by natural killer cells and cytotoxic T lymphocytes. When a class I MHC is coexpressed with viral antigens on virus-infected cells, cytotoxic target cells are signaled. Class II MHC molecules are markers indicating that a cooperative immune state exists between immunocompetent cells, such as between an antigen-presenting cell (APC) and a T-helper cell during the induction of Ab formation.
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