This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
[{'content': 'Which of the following complement components attaches to the crystallizable fragment of immunoglobulin M ? \nSelect and explain. \nA. C5b, B. C5a, C. Clqrs, D. C4b2a', 'role': 'user'}, {'content': "Answer: C \nExplanation: 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.", 'role': 'assistant'}]