Transplants from one individual to another member of the same species (‘allografts’) are recognised as foreign and rejected.

Gorer (1930) identified the antigens responsible for allograft rejection in inbred mice that led to the discovery of the major histocompatibility complex (MHC).

The MHC in humans is known as the human leukocyte antigen (HLA) complex.

HLA complex

Histocompatibility antigens mean cell surface antigens that evoke immune response to an incompatible host resulting in allograft rejection.

These alloantigens are present on surface of leucocytes in man and are called human leucocyte antigens (HLA) and the set of genes coding for them is named the HLA Complex.

The HLA complex of genes is located on short arm of chromosome 6 and is grouped in three classes

Class I

HLA-A, HLA-B and HLA-C

Class II

HLA-DR, HLA-DQ and HLA-DP (All of these are present within HLA-D region of HLA complex.)

Class III ( Complement loci encode for C2, C4 and Factor B of complement system and tumour necrosis factors (TNF) alpha and beta)

A locus is the position where a particular gene is located on the chromosome.

HLA loci are multiallelic i.e. the gene present on the locus can be any one of several alternative forms (alleles).

Each allele determines a distinct antigen. There are 24 alleles at HLA-A locus and 50 at HLA-B. HLA system is very pleomorphic. Every individual inherits one set of HLA-genes from each parent.

1.Class 1 MHC Antigens (A, B, C)

The MHC class I antigens are present on the surface of all nucleated cells. They are involved in graft rejection and cell mediated cytolysis. The cytotoxic T cells (CD8) recognise MHC class I antigens for their action.

2 Class II MHC Antigens (DR, DQ and DP)

They have a very limited distribution and are principally found on the surface of macrophages, monocytes, activated T-lymphocytes (CD4) and B-lymphocytes. They are primarily responsible for the graft-versus-host response and the mixed leukocyte reaction (MLR).

3.Class III MHC Antigens

Class III genes encode C2, C4 complement components of the classical pathway and properdin factor B of the alternative pathway.

Source – textbook of microbiology for dental students – harsh mohan