There are two classes of MHC molecules - class I and class II. The difference in the two classes is important.
All cells in the body have MHC class I protein molecules on the surface. Finger cells, liver cells, B cells, enteric cells - they all have MHC class I on the surface. Everyone has MHC class I proteins but only identical twins will have identical MHC class I. The genes for the MHC proteins are part of the immunoglobulin superfamily of genes and therefore show diversity between people.
Since everyone has a different MHC, this allows the body to tell which cells belong to it and which cells are foreign. The function of MHC-I is to sample the internal contents of the cell and show them to the immune system. If the contents of the cell are without foreign cells then no attraction of killer cells occurs. If, however, the cell is virally infected, then the killer cells can recognize the viral antigens presented by the Class-I MHC and the cells are killed. Unfortunately foreign MHC is seen by the immune system so that the cells in transplants activate the killer cells in a similar manner as virally infected cells. The end result is the same, cell death (and transplant rejection).
MHC-Class II
Class II MHC molecules are proteins expressed only by antigen presenting cells (APC). The function of these proteins is to present an antigen to T helper cells to activate an immune response, which provides both humoral (antibody) and cell mediated immunity.
The class II MHC consists of an alpha and a beta chain with a transmembrane segment to hold them on the surface of the cell. At the end farthest from the cell is a cleft where the processed antigen resides. The processed antigen consists of a small peptide of about 13-16 amino acids. Every MHC class II on the surface of a cell contains an antigen fragment because the MHC II is not expressed on the surface until it has antigen in the cleft. MHC-II picks up the antigen that has been ingested (phagocytosed) by the APC. It does not present self-antigen like MHC-I does.
OPSONIZATION
Opsonization is the process where particles such as microorganisms become coated with molecules, which allow them to bind to receptors on phagocytes. Antibodies (especially IgG) and complement proteins like C3b can opsonize and are therefore referred to as "opsonins.
The IgG antibodies bind to the antigens with the Fab region leaving the Fc region projecting out of the region. Phagocytes have Fc gamma receptors and therefore they can bind to the coated molecules and internalize them. The complement fragment, C3b, nonspecifically binds to foreign organisms. Phagocytes also have receptors for C3b on their surface. The antibodies and C3b tag the microorganisms for destruction by phagocytes.
