Which of the following complement components attaches to the crystallizable fragment of immunoglobulin M ?

Ans. is 'c' i.e., Clqrs Only IgM and IgG activate or fix complement the classic pathway. Of the IgGs, only IgG subclasses 1, 2 and 3 fix complement; IgG4 does not.CI, which is bound to a site in the Fc region, is composed of three proteins: Clq, CI r and Cls. Clq is an aggregate of polypeptides that bind to the Fc poion of IgG and IgM.The antibody-antigen immune complex bound to CI activates C I s, which cleaves C4 and C2 to form C4b2b. The latter is an active C3 convease, which cleaves C3 molecules into two fragments: C3a and C3b.