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As has been stated,
cellular immunity is mediated by T lymphocytes that can
recognize infected body cells, cancer cells, and the cells of
a foreign transplant. The control of cellular immune reactions
is provided by a linked group of genes, known as the major histocompatibility
complex (MHC). These genes code for the major histocompatibility
antigens, which are found on the surface of almost all 
nucleated somatic cells 
. The major histo-compatibility antigens were first discovered
on the leukocytes (white blood cells) and are, therefore, usually
referred to as the HLA (human leukocyte group A) antigens.
The advent
of the transplantation of human organs in the 1950s made the question
of tissue compatibility between donor and recipient of vital importance,
and it was in this context that the HLA antigens and the MHC were
elucidated. Investigators found that the MHC resides on the short
arm of chromosome 6, on four closely associated sites designated
HLA-A, HLA-B, HLA-C, and HLA-D. Each locus is highly polymorphic-i.e.,
each is represented by a great many alleles within the human gene
pool. These alleles, like those of the ABO blood group system,
are expressed in codominant fashion. Because of the large number
of alleles at each HLA locus, there is an extremely low probability
of any two individuals (other than siblings) having identical
HLA genotypes. (Since a person inherits one chromosome 6 from
each parent, siblings have a 25 percent probability of having
received the same paternal and maternal chromosomes 6 and thus
of being HLA matched.)
Catherine
Baker
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