ABO and H are the most important of the currently characterized blood group systems, since incompatibility between transfused red cells and recipient plasma leads to potentially devastating consequences. Those learning about this system spend lots of time memorizing biochemical details that can be overwhelming. In addition, exam-writers seem to enjoy asking questions about unusual entities in these systems that most blood bankers will never see in real life. Specific questions about two very rare situations with altered red blood cell appearances ("phenotypes"), known as "Acquired B" and "Bombay," are very commonly included on examinations.
In brief, ABO antigens on red blood cells are made in a sequential manner. First, long sugar chains attached to either lipids or proteins (glycolipids or glycoproteins, respectively) on the surface of the RBC must be modified through the work of an enzyme encoded by the H (FUT1) gene (chromosome 19) to display H antigen activity (for more details, please see the video above). Only then can the chain be further modified by the action of a second enzyme that adds a single sugar to change that H antigen into either an A or a B antigen. The alleles inherited at the ABO gene site on chromosome 9 (A, B, and/or O) determine which ABO antigens will be expressed on the red cell surface, but again, such a change ONLY happens if the precursor antigen (H) is made first.
The Bombay Phenotype