Qualitative and quantitative studies on IgG2 globulins in individual human sera with an antiserum capable of differentiating between Gm(n+) and Gm(n-) proteins.
A monkey antiserum was obtained by immunization with Gm(n+) IgG2 proteins. After it had been rendered specific for the IgG2 subclass, it was shown to contain antibodies of two specificities, i.e. anti-γ2 and anti-Gm(n). Precipitation and haemagglutination experiments, using unabsorbed antiserum and antiserum absorbed with Gm(n+) or Gm(n−) proteins, made it clear that the anti-Gm(n) was unable to form precipitates with Gm(n+) proteins without the assistance of anti-γ2 antibodies. Results obtained in radial immunodiffusion with serum samples of known Gm genotype showed that the antiserum differentiates three types of individuals: homozygous Gm(n− /n−), heterozygous Gm(n+ /n−) and homozygous Gm(n+ /n+). 
 
The amount of IgG2 present in serum samples from seventy-four individuals of variable Gm types was determined by comparison with a standard serum. It was found that the mean level of female sera (4·2 mg/ml) was higher than that of male sera (3·3 mg/ml). It was further shown that the IgG2 level was correlated with Gm(n). The total IgG2 content of individuals possessing the Gm(n) marker was higher than that of individuals of otherwise identical Gm type but lacking this factor. In individuals heterozygous for Gm(n) the level of Gm(n+) protein was significantly higher than that of Gm(n−) protein.