The immune response in the hamster. II. Studies on IgM

Hamster IgM has been isolated and characterized. The protein possessed unique antigenic determinants but also shared common determinants with the 7Sγ₁- and 7Sγ₂-globulin classes. These shared determinants were present on the F(ab′)₂ and Fab fragments of 7Sγ₂-globulin. The rapidly sedimenting (S_20,w = 20.7) IgM was dissociated into slowly sedimenting (≈ 7S) units after reduction and alkylation. Specific antibody formation in the IgM and IgG (7Sγ₁-globulin) classes appeared at similar times after immunization with protein antigens, although IgM antibody was only detectable for a short period. After immunization with antigen in Freund''s adjuvant, the serum concentration of IgM increased and remained at an elevated level even after disappearance of antibody to the immunizing antigen. Injection of adjuvant alone also increased the concentration of serum IgM, particularly after intraperitoneal administration of Freund''s incomplete adjuvant.