Why is IgM efficient at (i) preventing blood-borne infections and (ii) fixing complement, but (iii) less efficient than other antibody classes in inducing phagocytosis of immune complexes?

Question

What will be an ideal response?

Answer 1

(i) IgM is the first antibody to be produced by plasma cells during a primary antibody response and is secreted as a pentamer that circulates in the blood. Because of the large size of pentameric IgM, it does not penetrate effectively into infected tissues and is most effective against pathogens in the bloodstream. (ii) In the classical pathway of complement activation, at least two Fc regions are needed to bind C1, the first complement component in the pathway. A single pentameric molecule of IgM can thus initiate complement activation. In contrast, two IgG antibodies in close proximity to each other are needed to bind C1. (iii) Phagocytic cells carry both complement receptors and Fc receptors for IgG (Fc?R) and IgA (Fc?R), but there are no Fc receptors for IgM. Thus, immune complexes of IgM and antigen alone cannot be taken up by macrophages through Fc receptor-mediated endocytosis. An IgM:antigen:C3b complex can be phagocytosed by a macrophage after binding to complement receptors, but this is not as efficient as having both complement receptors and Fc receptors cooperating in inducing phagocytosis.