Final answer:
IgM is the class of antibodies that is primarily responsible for activating the complement system and causing cell clumping, with its pentameric structure allowing it to effectively bind and agglutinate cells.
Step-by-step explanation:
The class of antibodies that plays a role in activating the complement system and clumping cells is IgM. IgM antibodies are the first type produced during the primary response to an antigen. When IgM molecule binds to cellular antigens, it leads to activation of the complement system and cell lysis. Also, the pentameric structure of IgM allows it to bind multiple identical antigens, enhancing its ability to clump cells together, which is known as agglutination.
While IgG antibodies can also activate complement proteins, they are not as effective as IgM in this function. IgG is a monomeric antibody responsible for clearing pathogens from the blood, and it is the main antibody in late primary and secondary immune responses. In contrast, IgE and IgD are present in smaller quantities and play different roles in the immune system, with IgE primarily associated with allergic responses and IgD with antigen recognition and maturation of B cells.