Which pair represents two different immunoglobulin allotypes?

• A. IgG1m3 and IgG1m17
• B. IgM and IgG
• C. IgM1 and IgM2
• D. Anti-human IgM and anti-human IgG

Answer: (A)

Allotypes are inherited differences in the constant regions of antibodies within the same isotype. They reflect genetic variation in the Fc region, not changes in the antigen-binding sites, and they’re identified by specific anti-allotype antibodies.

The pair IgG1m3 and IgG1m17 shows two distinct allotypes on the same antibody class, both being IgG1 but carrying different allotypic determinants (m3 vs m17) on the gamma-1 heavy chain. That directly exemplifies two different allotypes.

The other options mix different isotypes (IgM vs IgG), propose what would be atypical or nonstandard allotype designations for IgM, or refer to reagents (anti-human IgM/IgG) rather than immunoglobulins with allotypic variation. Hence they do not represent two different allotypes of the same antibody class.