Why are monoclonal antibodies unsuitable for complement fixation tests?

Monoclonal antibodies are specifically designed to bind to a single type of antigen with high affinity. However, in the context of complement fixation tests, the way they interact with the complement system makes them unsuitable for this application.

Complement fixation tests rely on the ability of antibodies to activate the complement system in the presence of antigen. When an antigen-antibody complex forms, the complement proteins are activated, leading to detectable lysis of target cells or other visible changes. Monoclonal antibodies, while effective at binding their specific antigens, often do not activate the complement pathway efficiently due to their design and the nature of the interactions involved.

This means that even if a monoclonal antibody binds to its target antigen, it may not effectively initiate the complement cascade needed for the test to be useful. Thus, the inactivation of the complement pathway is why monoclonal antibodies are considered unsuitable for complement fixation tests.