Discussion
Studying the immunogenetic aspect of a disease is most useful,
not only for identifying the mode of inheritance of a particular
disease process but also for understanding the
immunopathogenic mechanisms underlying it. The discovery
of HLA associations with a specific disease implies that at
least part of the genetic basis lies in the MHC, thereby indicating
the possibility of determining its etiology (24). A close
study of different populations has shown that certain
combinations of HLA alleles occur together more often than
would be expected on the basis of their individual gene frequencies
(24). This nonrandom association of the alleles of
2 HLA loci found together on the same HLA haplotype is
termed “linkage disequilibrium” and is expressed in terms of
delta values. One possible explanation for the association of
HLAs with a disease is the existence of linkage disequilibrium
between the alleles. Incidentally, most of the diseases that
have shown strong associations with HLAs—such as various
autoimmune and rheumatologic disorders—have unknown
etiology and mode of inheritance.