This is a term used to describe a pair (and occasionally more than a pair) of antigens that are coded by different alleles of a single gene. For example, in the MNS blood group system, a person can inherit an allele from his or her parent that codes for either an M antigen or an N antigen (but not both in one gene). So, you could say that the M antigen is antithetical to the N antigen, since an individual allele at the MN gene site (known as GYPA) can only be for M or N, not both (the same logic applies to other antithetical pairs such as K and k, Jk^a and Jk^b, C and c, S and s, to name a few). NOTE: This definition does NOT mean that only one of an antithetical pair of antigens is ever carried on a single RBC! In our example above, a person could certainly be M-positive AND N-positive (M+N+), but the point is that the gene for M is inherited from one parent and that for N from the other parent.