The structure and function of the molecules that carry human red blood cell and platelet antigens

A number of molecules on the surface of red blood cells (RBCs) and platelets express antigenic activity. Various biochemical and molecular approaches have been used to determine the structure and possible function that these molecules have for their respective cell types. The existence of variant molecules and null phenotypes and the immunological response to these antigens have aided in the analysis of the structure and function relationships of these molecules. A comparison of the sequence to moieties of known function and the presence of functional domains for many of the molecules allows for a prediction of their function. The proposed function of the molecules that express RBC and platelet antigens includes membrane structure, transporter or channel formation, receptor/ligand signaling or adhesion, enzyme activity, and glycocalyx formation. However, the function of some of these molecules is not known, and many of the variant antigens do not show an obvious functional difference. For unknown reasons, some of these molecules are exceptionally polymorphic and the elucidation of the precise role that these polymorphisms play in structure and function is hindered by limitations in the in vitro and ex vivo analyses and access to precursor cell types. The objective of this review is to define the structure and function of those molecules that express RBC and platelet antigens and the significance, if any, that polymorphisms have for these molecules.