Which two classes of antibodies are known to fix or activate a complement?

• A. IgA and IgE
• B. IgG and IgD
• C. IgG and IgM
• D. IgM and IgE

Answer: (C)

The correct answer, which includes IgG and IgM, is significant because these two classes of antibodies are particularly effective at activating the complement system. The complement system is a crucial part of the immune response; it consists of a series of proteins that, when activated, can enhance the ability of antibodies and phagocytic cells to clear pathogens, promote inflammation, and directly lyse pathogens.

IgM is highly efficient at complement activation due to its pentameric structure when it is secreted. This structure allows it to bind multiple antigens and initiate the complement cascade effectively. IgG can also activate the complement system, primarily through its subclasses (IgG1, IgG2, and IgG3), which can bind to antigens and interact with complement proteins, enhancing immunity.

In contrast, other antibody classes listed do not have the same capability to activate complement. For example, IgA primarily operates at mucosal surfaces and plays a role in preventing pathogen entry but is less involved in complement activation. IgE is associated with allergic reactions and defense against parasitic infections but does not activate complement effectively. Lastly, IgD's role is not well understood in the context of complement activation and primarily functions in B cell receptor signaling.

Therefore, the ability