Which of the following antibodies can activate complement?

The correct answer is that IgA, IgG, and IgM are all capable of activating the complement system. The complement system is an important part of the immune response, helping to clear pathogens and facilitate inflammation.

IgM is particularly effective at complement activation because it exists primarily as a pentamer, which allows it to bind multiple antigen molecules simultaneously. This extensive cross-linking can effectively trigger the classical pathway of complement activation. IgG also activates complement, particularly the IgG1 and IgG3 subclasses, as it can bind to antigens on pathogens and then activate the complement system.

While IgA plays a more pivotal role in mucosal immunity, recent studies have shown that certain subclasses of IgA can also participate in complement activation, albeit less effectively than IgM and certain IgG subclasses.

The other answer choices include antibodies that have limited or no ability to activate complement, or combinations that do not represent the full capacity of the complement system activation. For instance, IgE is primarily involved in allergic responses and does not activate complement effectively, while IgD's role in the immune response is not directly linked to complement activation.