Question

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

Short answer

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.

Step-by-step solution

Define complement fixation

Complement fixation is a process of the immune system involving the activation of the complement system, a group of proteins that enhance the ability of antibodies and immune cells to fight and clear pathogens from the body. When activated, these proteins assemble on the surface of a target, such as a pathogen or foreign substance, and ultimately lead to the destruction of the target.

Explain the stimulus for complement fixation

The stimulus for complement fixation is the presence of an immune complex, which occurs when an antibody (a protein produced by the immune system in response to an invading foreign substance) binds to an antigen (a molecule recognized as foreign by the immune system). The binding of the antibody to the antigen initiates the activation of the complement system, leading to complement fixation.

Describe the steps involved in complement fixation

There are three main pathways for the activation of the complement system in response to an immune complex: the classical pathway, the alternative pathway, and the lectin pathway. These pathways converge at a certain point, leading to the formation of the membrane attack complex (MAC). The MAC creates a pore on the surface of the target cell or foreign substance, causing an influx of fluid and the eventual lysis (disintegration) of the target.

Explain the destruction of cellular antigens

When the complement system is activated in response to a cellular antigen (such as a pathogen or an infected body cell), the complement proteins assemble on the surface of the targeted cell. The formation of the MAC creates a pore in the cell membrane, leading to the influx of fluid and the eventual osmotic lysis (bursting) of the targeted cell. The rupture of the targeted cell releases its contents, which can be cleared by the immune system.

Explain the destruction of noncellular antigens

Noncellular antigens, such as viral particles or protein aggregates (e.g., amyloid fibrils), can also be targeted by the complement system. In this case, the binding of antibodies to the noncellular antigen forms an immune complex, which initiates the activation of the complement system. Complement proteins assemble on the surface of the noncellular antigen, and the formation of the MAC causes damage and disintegration of the target structure. The fragmented components of the noncellular antigen can then be cleared by the immune system.