Which pathway of the complement system is activated by the formation of an antigen-antibody complex?

• A. Alternative pathway
• B. Lectin pathway
• C. Classical pathway
• D. Immunoglobulin pathway

Answer: (C)

The classical pathway of the complement system is specifically activated by the formation of immune complexes, which are comprised of antigens bound to antibodies. When an antibody, typically an immunoglobulin of the IgG or IgM type, binds to an antigen, it undergoes a conformational change that allows it to interact with and activate the first component of the complement system, known as C1. This leads to a cascade of additional complement proteins being activated, resulting in processes such as opsonization of pathogens, formation of the membrane attack complex, and enhancement of inflammation.

In contrast, the alternative pathway is activated spontaneously and does not require antibodies, while the lectin pathway is triggered by the binding of mannose-binding lectin to specific sugars on microbial surfaces. The so-called “immunoglobulin pathway” is not a recognized pathway in the classical descriptions of the complement system, making it an incorrect choice. Thus, the classical pathway’s dependence on antigen-antibody complexes underscores its role in the adaptive immune response.