Human IgG3 allotypes, with special reference to a new allotype related to G3m(g) (G3m21).

The IgG3 allotype described as L1 (Blanc et al., 1976) occurs on the CH3 region of G3m(g) proteins, in contrast to G3m(g) that is known to be present on the CH2 region. G3m(g) and L1 are, as a rule, present on the same gamma3 heavy chain, just like the G3m(b) subspecificities of the CH2 region, (b1) and (b4), with those of the CH3 region (b0), (b3) and (b5). Several families were investigated that showed inheritance of rare combinations of IgG3 allotypes. The data obtained are suggestive for notation of L1 as (g5), since L1 probably occupies a position antithetical to (b5). The relation of amino acid substitutions to allotypes and isoallotypes is discussed.