Which pathway in the complement system is activated by an antigen-antibody response?

The pathway in the complement system that is activated by an antigen-antibody response is the classical pathway. This pathway plays a critical role in the immune response, particularly in enhancing the ability of antibodies and phagocytic cells to clear pathogens and damaged cells.

When an antibody binds to an antigen on the surface of a pathogen, it undergoes a conformational change that exposes a specific region of the antibody used for complement activation. This region interacts with the C1 complex, which is the first component of the classical pathway. The binding initiates a cascade of events, leading to the activation of several complement proteins. This cascade ultimately results in various immune responses, such as opsonization of pathogens, recruitment of inflammatory cells, and formation of the membrane attack complex, which can directly lyse pathogens.

In contrast, the lectin pathway is activated by the binding of mannose-binding lectin to specific carbohydrates on the surface of pathogens, while the alternative pathway is initiated by the spontaneous hydrolysis of complement component C3 and can be activated without antibody involvement. These pathways serve as important mechanisms in the complement system, but they do not rely on the antigen-antibody interactions that characterize the classical pathway.