Final answer:
C3b is the main opsonin deposited onto a pathogen surface from the complement system and plays a crucial role in the immune response by marking pathogens for phagocytosis and initiating further complement activation.
Step-by-step explanation:
The main opsonin deposited onto a pathogen surface from the complement system is C3b. Once the complement system is activated, either through the classical pathway, which requires antibodies, or through the alternate pathway, which is antibody-independent, C3 is split into C3a and C3b. The C3b fragment binds to the surface of the pathogen, effectively marking it for phagocytosis by cells such as neutrophils and macrophages. This opsonization process enhances the ability of the immune system to eliminate pathogens.
Opsonins, like C3b, are crucial for the immune response, as they not only flag pathogens for destruction but also activate the rest of the complement cascade, leading to the formation of the membrane-attack complex (MAC). The MAC disrupts the osmotic balance of pathogens, such as bacteria, leading to their death.