The major histocompatibility complex (MHC)
is a set of cell surface proteins essential
for the acquired immune system 
to recognize foreign molecules in vertebrates,
which in turn determines histocompatibility.
The main function of MHC molecules is 
to bind to peptide fragments derived from
pathogens and display them on the cell surface
for recognition by the appropriate T-cells.
MHC molecules mediate interactions of leukocytes,
also called white blood cells (WBCs), which
are immune cells, with other leukocytes or
with body cells.
The MHC determines compatibility of donors
for organ transplant, as well as one's susceptibility
to an autoimmune disease via crossreacting
immunization.
In humans, the MHC is also called 
the human leukocyte antigen (HLA).
In a cell, protein molecules of the host's
own phenotype or of other biologic entities
are continually synthesized and degraded.
Each MHC molecule on the cell surface displays
a molecular fraction of a protein, called
an epitope.The presented antigen can be eitherself
or non-self, thus preventing an organism's
immune system targeting its own cells.
In its entirety, the MHC population is like
a meter indicating the balance of proteins
within the cell.
The MHC gene family is divided into three
subgroups: class I, class II, and class III.
Class I MHC molecules have �2 subunits so
can only be recognised by CD8 co-receptors.
Class II MHC molecules have no �2 subunits
so can be recognised by CD4 co-receptors.
In this way MHC molecules chaperone which
type of lymphocytes may bind to the given
antigen with high affinity, since different
lymphocytes express different TCR co-receptors.
Diversity of antigen presentation, mediated
by MHC classes I and II, is attained in at
least three ways: an organism's MHC repertoire
is polygenic (via multiple, interacting genes);
MHC expression is codominant (from both sets
of inherited alleles); MHC gene variants are
highly polymorphic (diversely varying from
organism to organism within a species).[3]
Major histocompatibility complex and sexual
selection has been observed in male mice making
mate choices of females with different MHCs
and thus demonstrating sexual selection.
Also, at least for MHC I presentation, there
has been evidence of antigenic peptide splicing
which can combine peptides from different
proteins, vastly increasing antigen diversity.
MHC is the tissue-antigen that allows the
immune system (more specifically T cells)
to bind to, recognize, and tolerate itself
(autorecognition).
MHC is 
also the chaperone for intracellular peptides
that are complexed with MHCs and presented
to TCRs as potential foreign antigens.
MHC interacts with TCR and its co-receptors
to optimize binding conditions for the TCR-antigen
interaction, in terms of antigen binding affinity
and specificity, and signal transduction effectiveness.
Essentially, the MHC-peptide complex is a
complex of autoantigen/alloantigen.
Upon binding, T cells should in principle
tolerate the auto-antigen, while activate
when exposed to the allo-antigen.
Disease states occur when this principle is
disrupted.
Antigen presentation: MHC molecules bind to
both T cell receptor and CD4/CD8 co-receptors
on T lymphocytes, and the antigen epitope
held in the peptide-binding groove of the
MHC molecule interacts with the variable Ig-Like
domain of the TCR to trigger T-cell activation
Autoimmune reaction: Having some MHC molecules
increases the risk 
of autoimmune diseases more than having others.
HLA-B27 is an example.
It is unclear how exactly having the HLA-B27
tissue type increases the risk of ankylosing
spondylitis and other associated inflammatory
diseases, but mechanisms involving aberrant
antigen presentation or T 
cell activation have been hypothesized.
Tissue allorecognition: MHC molecules in complex
with 
peptide epitopes are essentially ligands for
TCR.
T cells become activated by 
binding 
to the peptide-binding grooves of any MHC
molecule that T cells were not entrained 
to recognize during thymus positive selection.
