HUMAN LEUKOCYTE ANTIGEN COMPLEX (HLA)

It is the human major histocompatibility complex. The genetic loci are located on the short arm of chromosome 6. It is made up of several closely linked genetic loci in three classes. Fig. 40.

Class I HLA loci - HLA - A, HLA - B and HLA - C. They recognise foreign antigens by cytotoxic T lymphocytes and are particularly important in graft rejection.

Class II HLA loci - HLA - DR, HLA - DP and HLA - DQ. They are involved in recognition of foreign antigens by helper T lymphocytes.

Class III HLA loci - C2, C4S, C4F and BF. They code for a few complement loci.

As they are closely linked, they are transmitted as a unit, called a haplotype in a codominant fashion. Fig. 41. Most HLA genes produce cell surface antigens. In organ transplantations, the greater the similarity between HLA types of the donor and recipient , the better the chance of avoiding rejection.