Complex sentence: Phagocytic cells do not have an FC receptor for IgM, making IgM ineffective in assisting phagocytosis; however, IgM is extremely efficient at activating complement and is therefore considered an opsonin.
Split into simpler sentences: Phagocytic cells do not have an FC receptor for IgM, making IgM ineffective in assisting phagocytosis. However, IgM is extremely efficient at activating complement and is, therefore, considered an opsonin.