What is the stimulus for complement fixation? How does this process destroy cellular antigens and noncellular antigens? (p. 333)

Answer: Complement fixation is a process of the immune system that involves the activation of the complement system, a group of proteins that enhance the immune response against pathogens. It occurs upon the formation of an immune complex, which is the binding of an antibody to an antigen. There are three main activation pathways: classical, alternative, and lectin, which lead to the formation of the membrane attack complex (MAC). In case of cellular antigens, the MAC creates a pore in the cell membrane, causing osmotic lysis and destruction of the targeted cell. For noncellular antigens, the MAC damages and disintegrates the target structure, and the fragmented components are cleared by the immune system.