Major Histocompatibility Complex

Major histocompatibility complexes (MHC) are molecules located in cell membranes of cells. MHC types relevant in acquired immunity include MHC I and MHC II types.

MHC I
MHC I molecules are found on the membranes of all nucleated cells (which excludes red blood cells). MHC I is used to present epitopes consisting of internally broken down molecules. If a cell is infected, it has the possibility of breaking down an antigen into an foreign epitope. Cells present epitopes (not entire antigens) along with MHC I on their surfaces. TCD8 cells, which bind to MHC I with their CD8 receptors, are able to probe the surfaces of MHC I-presenting cells for foreign epitopes using TCRs (T-cell receptors). The foreign epitope is then recognized by TCD8 cells as foreign and an immune response is able to be mounted.

MHC II
MHC II molecules are also found on the membrane but are limited mainly to phagocytotic leukocytes such as macrophages, dendritic cells, and B-lymphocytes. These phagocytotic leukocytes are also called professional APCs. MHC II are present in addition to MHC I, so any cell with MHC II also should have MHC I (since it is present in all nucleated cells).