IMMUNOLOGY

Detailed explanation-1: -The complement system is made up of a large number of distinct plasma proteins that react with one another to opsonize pathogens and induce a series of inflammatory responses that help to fight infection. A number of complement proteins are proteases that are themselves activated by proteolytic cleavage.

Detailed explanation-2: -First, complement factors directly promote vascular cell dysfunction through their specific receptors in effector cells. However, complement activation mediates immune cell recruitment and induces inflammatory factor production in these cells.

Detailed explanation-3: -(1) A pathogen coated in C3b binds to complement receptor 1 (CR1) expressed on the surface of phagocytes (including APCs). These cells then easily engulf and destroy the invader. The C3b is said to be acting as an opsonin in this case because it enhances the ability of an entity to be phagocytosed.