Which two antibodies are primarily responsible for initiating the classical complement pathway?

The classical complement pathway is a critical component of the immune response, and it is primarily initiated by the binding of complement proteins to antibodies that are bound to antigens. The antibodies that play a pivotal role in this activation are IgG and IgM.

IgG is the most abundant antibody in circulation and is highly effective in opsonizing pathogens, meaning it tags them for destruction. It can bind to bacterial surfaces and form complexes with antigens. When IgG binds to an antigen, it can activate the complement system, leading to a cascade of events that help eliminate the pathogen.

IgM, on the other hand, is the largest antibody in terms of size and is particularly efficient in the early stages of an immune response. Its pentameric form (five monomers joined together) allows it to bind multiple antigens simultaneously, which facilitates its interaction with the complement system. Once IgM binds to an antigen, it changes conformation, exposing sites for the complement proteins to bind and initiate the complement cascade.

Together, IgG and IgM are crucial for the initiation of the classical complement pathway, thereby enhancing the ability of the immune system to respond to infections. The presence of these antibodies in immune complexes allows complement components, such as C1,