Significant progress was made recently in defining the structure of the highly polymorphic B-G antigens encoded within the major histocompatibility complex of the chicken. It has been determined that B-G antigens are a family of molecules, divergent members of which are expressed on cells in several different tissues in addition to erythrocytes, including immune system cells. Determination of the sequences of some of the B-G family members has revealed the structural basis of B-G polymorphism, including the presence of hypervariable sequence diversity within an immunoglobulin v-like domain. B-G cytoplasmic tails are made up of heptad repeats of diverse sequence. B-G antigens have been found on the epithelia in the gut, cells in the liver, thrombocytes (highly phagocytic cells in birds), some lymphocytes, and cells within the stroma of the bursa and thumus. The objective of this project is to attempt to define the function (or functions) of the B-G antigens in light of the newly recognized sites where B-G molecules are now known to be expressed. The B-G antigens have long been known as blood group antigens of chickens. Until recently, the B-G antigens were thought to be expressed exclusively on red blood cells. They are now known to be expressed in various cell types and tissues throughout the chick, at various stages of development. Furthermore, they have been shown through DNA sequencing work to have structural resemblance to immunoglobulin genes. These new observations have led to the hypothesis that the B-G antigens may play some role in the chicken immune system. The goal of this study is to explore this hypothesis. The results of this study have the potential to shed new light on how the immune system of the chicken operates.
