Both IgG and IgM activate complement by the classic pathway, while IgA activates it by the alternative pathway. Neither IgD nor IgE can activate complement. Complement is a system of several proteins that is activated by either an immune or a nonimmune pathway. Both of these pathways result in the production of many biologically active components that cause cell lysis and death. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition