Which immunoglobulin(s) help(s) initiate the classic complement pathway?

The initiation of the classical complement pathway is primarily attributed to the binding of specific immunoglobulins to antigens. In particular, both IgM and IgG can initiate this pathway effectively.

IgM is especially noteworthy because it is a pentamer in its secreted form, allowing it to cross-link multiple antigens. This structural configuration makes IgM a powerful activator of complement because it can interact with the C1 complex more efficiently than monomeric forms of immunoglobulins. When IgM binds to an antigen, it undergoes a conformational change that facilitates the binding of C1q, the first component of the classical complement pathway, leading to a cascade of events that results in complement activation.

IgG, on the other hand, primarily exists as a monomer but is also very effective in activating the classical complement pathway, particularly in its subclasses IgG1 and IgG3. These subclasses possess the ability to bind to the C1 complex similarly to IgM, although they may require a higher concentration in comparison to IgM for optimal activation.

The combination of both IgG and IgM in this context confirms the importance of these immunoglobulins in initiating the classical complement pathway. Thus, the correct answer encompasses both