Final answer:
The opsonins for CR3 and CR4 are mainly IgG antibodies and complement factors, which facilitate phagocytosis of pathogens by binding to these complement receptors on phagocytes.
Step-by-step explanation:
The opsonins for CR3 and CR4, which are complement receptors on phagocytic cells like macrophages and neutrophils, are immune molecules that promote phagocytosis.
One of the primary opsonins that can bind to these receptors is IgG antibodies. These antibodies attach their Fab regions to specific antigens on the surface of pathogens and subsequently bind their Fc regions to CR3 and CR4 on phagocytes, prompting the engulfment and destruction of the pathogen.
Complement factors, such as those involved in the complement cascade, also serve as opsonins and can enhance the binding of pathogens to CR3 and CR4. This process not only includes opsonization but also additional immune responses such as inflammation and cell lysis through the activation of the complement system.