Which immunoglobulin class is most efficient at initiating the classical complement pathway?

Which immunoglobulin class is most efficient at initiating the classical complement pathway?

• A. IgA
• B. IgG
• C. IgM
• D. IgD

Answer: (C)

The classical complement pathway is triggered when C1q binds to the Fc region of antibodies that are attached to an antigen. IgM is especially effective here because it is a pentamer, presenting multiple Fc regions together. When an IgM molecule binds to an antigen, it undergoes a conformational change that exposes Fc sites in a way that readily recruits and activates C1, starting the cascade. This means a single IgM bound to its antigen can initiate complement activation efficiently. IgG can also activate the pathway, but it generally requires two or more IgG molecules in close proximity to provide enough Fc material for C1q binding, so it’s less efficient than IgM. IgA is not efficient at triggering the classical pathway and is more involved in mucosal defense, and IgD is not a major driver of complement activation.

The classical complement pathway is triggered when C1q binds to the Fc region of antibodies that are attached to an antigen. IgM is especially effective here because it is a pentamer, presenting multiple Fc regions together. When an IgM molecule binds to an antigen, it undergoes a conformational change that exposes Fc sites in a way that readily recruits and activates C1, starting the cascade. This means a single IgM bound to its antigen can initiate complement activation efficiently. IgG can also activate the pathway, but it generally requires two or more IgG molecules in close proximity to provide enough Fc material for C1q binding, so it’s less efficient than IgM. IgA is not efficient at triggering the classical pathway and is more involved in mucosal defense, and IgD is not a major driver of complement activation.