Final answer:
The antibodies besides IgG that can fix complement are IgM. IgM is very efficient in activating the complement system in comparison to IgG.
Step-by-step explanation:
In addition to IgG, the antibodies that can fix complement are IgM. Both IgG and IgM have crucial roles in the immune system. IgG is significant in late primary responses and secondary responses, as it clears pathogens from bloodstream and can activate complement.
On the other hand, IgM, which forms pentamers, is capable of activating the complement system even more efficiently than IgG. Other antibody classes such as IgA, IgD, and IgE have different functions within the immune response but do not fix complement in the same way as IgG and IgM.