The Major Histocompatibility Molecules
Bind Antigenic Peptides
• The major histocompatibility complex (MHC) is a large genetic complex
with multiple loci. The MHC loci encode two major classes of membrane-
bound glycoproteins: class I and class II MHC molecules. As noted above,
TH cells generally recognize antigen combined with class II molecules,
whereas TC cells generally recognize antigen combined with class I
molecules.
• MHC molecules function as antigen-recognition molecules, but they do
not possess the fine specificity for antigen
characteristic of antibodies and T-cell receptors. Rather, each
MHC molecule can bind to a spectrum of antigenic peptides
derived from the intracellular degradation of antigen molecules.

• In both class I and class II MHC molecules the distal regions (farthest from the membrane) of
different alleles display wide variation in their amino acid sequences. These variable regions form a
cleft within which the antigenic peptide sits and is presented to T lymphocytes
• Different allelic forms of the genes encoding class I and class
II molecules confer different structures on the antigen-binding cleft with different specificity. Thus
the ability to present
an antigen to T lymphocytes is influenced by the particular
set of alleles that an individual inherits

Complex Antigens Are Degraded (Processed) and Displayed (Presented) with
MHC Molecules on the Cell Surface
• In order for a foreign protein antigen to be recognized by a T
cell, it must be degraded into small antigenic peptides that
form complexes with class I or class II MHC molecules. This
conversion of proteins into MHC-associated peptide
fragments is called antigen processing and presentation

• Whether a particular antigen will be processed and presented
together with class I MHC or class II MHC molecules appears
to be determined by the route that the antigen takes to enter
a cell