The first isoallotype of human IgA proteins. An antigenic determinant occurring as allotype in the IgA2 subclass and as isotype in the IgA1 subclass.

An antigenic determinant that occurs on all IgA proteins except on IgA2 proteins of the A2m2 allotype is described. The determinant is an isoallotype, being an isotype of IgA1 and at the same time an allotype of IgA2 proteins. It is proposed that the symbol nA2m2 be used for the new determinant, where n stands for isoallotype (formerly nonmarker) and A2m2 refers to the allotype to which it is antithetically related.