f Antigen

NOTE: Before you read this entry, you should be very familiar with the genetics and terminology in the Rh Blood Group System. If you don’t feel comfortable with “Rh haplotypes” and what they mean, you should read my blog post on Rh terminology first, then come back here.

The antigen known as “f” has historically been described as a “compound antigen” in the Rh blood group system. The f antigen is present when a person inherits an allele of the RHCE gene that codes for both the c and e antigens (specifically, the RHCE*ce allele of the RhCE gene), and absent if the person does not inherit that allele (i.e., if the person inherits only some combination of the other three RHCE alleles: RHCE*Ce, RHCE*cE, or the very uncommon RHCE*CE). With me so far? Hang in there, and go review the blog post above if that doesn’t make sense.

When we use the “Modified Wiener” shorthand terminology to describe someone’s Rh genotype (you know, the “R₁, R₂, R₀, r” stuff you learned in the post I keep bugging you about!), this means that anyone who inherits either the Rh haplotype R₀ or r is f-positive, since both haplotypes include the RHCE*ce allele. Since most D-negative people have either the rr genotype (most common) or the rr’ genotype, the vast majority of D-negative (“Rh-negative”) individuals are f-positive.

People who lack the f antigen are almost always D-positive, with genotypes like R₁R₁, R₁R₂, or R₂R₂ (to name a few). Those who lack the antigen, not surprisingly, can make an antibody against the f antigen. Anti-f acts much like other Rh antibodies, with evidence of hemolytic disease of the fetus/newborn (HDFN) in f-positive babies born to f-negative moms with anti-f, and possible hemolytic transfusion reactions if someone with anti-f is exposed to f-positive red blood cell transfusions.

Note that the antigen is not really “compound”, in that it isn’t formed by the mere presence of c and e on the same red cell, but rather by the action of the RhCE*ce allele that also encodes both c and e. So, it might be better to say that RHCE*ce codes for c, e, and f.

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