A great deal of work has been performed by various experts in attempts to understand the chemical characteristics of the red cell antigens. Apparently the facts remain sketchy. However, it is generally accepted that red cell antigens are glycolipids or proteins. The specificity of the antigens is determined by the sequential addition of sugar residues to a common “precursor substance”. This has proved to be the case in all blood group antigens studied so far. The precursor substance is composed of four sugar molecules in which two are known as D-galactose (GAL), one is N-acetyl-galactosamine (GAL NAc) and the last is N-acetyl-glucosamine (GNAc). Two types of precursor substance have been identified, known as Type 1 and Type 2 chains. These chains differ in the linkage of the terminal galactose molecule to the subterminal N-acetyl-glucosamine. In Type 1 chains the linkage is beta (1-3), whereas in Type 2 chains the linkage is beta (1-4) (Chase snd Morgan, 1991; Painter et al, 1963; Watkins, 1966).
The addition of another sugar to this basic precursor determines the specificity of the antigen with the rest of the polysaccharide chain remaining unchanged. Knowledge of the chemistry of blood group antigens has come mainly from work on the body secretions (semen, tears, saliva and cyst fluids), since blood group substances on the red cells are present in relatively small quantities and some of them are soluble only in alcohol. In secretions, these substances occur in much larger quantities and are soluble in water. It has been established that the same sugars are present on the red cells, though on the red cells they are bound through sphingosine to fatty acid moieties. (compounds of this type are known as glycolipids)