Bookshelf

Most species have genetic polymorphisms of the constant regions of their immunoglobulin heavy- and light-chain genes; these are known as allotypes (see Section 4-20). In rabbits, for example, all of the B cells in an individual homozygous for the a allele of the immunoglobulin heavy-chain locus (Igh) will express immunoglobulin of allotype a, whereas in an individual homozygous for the b allele all the B cells make immunoglobulin of allotype b. In a heterozygous animal, which carries the a allele on one of the Igh chromo-somes and the b allele on the other, individual B cells can be shown to carry either a-type or b-type immunoglobulin, but not both. This allelic exclusion reflects productive rearrangement of only one of the two parental Igh alleles.

Differences between constant regions due to usage of different C-region genes are called isotypes; differences due to different alleles of the same C gene are called allotypes; differences due to particular rearranged V_H and V_L genes are called idiotypes.

The subunit structure and binding properties of these receptors and the cell types expressing them are shown. The complete multimolecular structure of most receptors is not yet known but they might all be multichain molecular complexes similar to the Fcε receptor I (FcεRI). The exact chain composition of any receptor can vary from one cell type to another. For example, FcγRIII in neutrophils is expressed as a molecule with a glycophosphoinositol membrane anchor, without γ chains, whereas in NK cells it is a transmembrane molecule associated with γ chains as shown. The FcγRII-B1 differs from the FcγRII-B2 by the presence of an additional exon in the intracellular region. This exon prevents the FcγRII-B1 from being internalized upon cross-linking. The binding affinities are taken from data on human receptors. *Only some allotypes of FcγRII-A bind IgG2. †In these cases Fc receptor expression is inducible rather than constitutive. ‡In eosinophils, the molecular weight of CD89α is 70–100 kDa.