What is the purpose of C3a, C4a, and C5a, the split products of the complement cascade?

C3a, C4a, and C5a are known as anaphylatoxins, which are split products of the complement cascade. Their primary function is to enhance the immune response by increasing vascular permeability, promoting smooth muscle contraction, and triggering the release of histamine from basophils and mast cells. This action leads to inflammation and aids in directing immune cells to the site of infection or injury.

In this context, the correct answer highlights these roles, focusing on the effects these molecules have on blood vessels and immune cell behavior. By increasing vascular permeability, they allow for more immune cells to infiltrate tissues, which is vital for combating pathogens and facilitating the inflammatory response.

The other choices, while involving important immune mechanisms, do not accurately describe the primary role of C3a, C4a, and C5a. For instance, binding to lymphocyte receptors to release cytotoxic substances is a function more associated with different immune signals rather than anaphylatoxins. Similarly, while C3b can play a role in opsonization by binding to macrophages, this is not the case for the anaphylatoxins. Lastly, regulating and degrading membrane cofactor proteins pertains to a different aspect of complement regulation, unrelated