Which immunoglobulins fix complement?

Which immunoglobulins fix complement?

• A. IgG and IgM
• B. IgA and IgE
• C. IgD and IgM
• D. IgA and IgG

Answer: (A)

Complement fixation happens when antibody–antigen complexes activate the classical pathway by binding C1q. This is most effective with IgM, which as a pentamer presents a single Fc region that readily activates C1q, and with IgG, whose Fc region can activate C1q when bound to antigen (especially certain subclasses). IgA and IgE do not fix complement in the classical pathway, and IgD mainly serves as a B cell receptor and does not fix complement. So the immunoglobulins that fix complement are IgG and IgM.

Complement fixation happens when antibody–antigen complexes activate the classical pathway by binding C1q. This is most effective with IgM, which as a pentamer presents a single Fc region that readily activates C1q, and with IgG, whose Fc region can activate C1q when bound to antigen (especially certain subclasses). IgA and IgE do not fix complement in the classical pathway, and IgD mainly serves as a B cell receptor and does not fix complement. So the immunoglobulins that fix complement are IgG and IgM.