Most cells of the body have on their surfaces proteins called HLA (human leukocyte antigens). These are depicted in the figure below. They serve a very important role in alerting the immune system to the presence of antigens derived from foreign sources such as viruses or altered self such as cancers. HLA proteins must be able to capture and present antigens of every conceivable type. To do so efficiently, each cell has a complement of slightly different HLA molecules each of which specializes in interacting with different types of antigens. Interestingly, the complement of HLA molecules found in one individual is different than that found in others. Thus, the complement of HLA molecules is a virtual identity code for the individual
When an organ from an unrelated donor is transplanted into a recipient, the recipient recognizes the presence of the donors HLA complement as different from their own and generates an immune response which can lead to rejection of the organ.
HLA Antigens: serve as antennae to recognize foreign germs or viruses entering the body. They communicate this information to the white blood cells to initiate an immune response.
A pregnancy is also recognized as a foreign being because the fetus shares some of the HLA molecules of the father. In the case of a normal pregnancy, however, instead of generating an immune response leading to rejection of the fetus, the mothers immune system first recognizes the fetus as foreign but develops tolerance to the fetal HLA molecules. The mechanism for this tolerance is a very active area of investigation. It is believed that recognition of the paternal HLA molecules leading to the development of tolerance may be deficient in some women with recurrent pregnancy loss.