Question

How does the complement system provide immunity to the body?

Answer 1

The complement system is a group of about 30 plasma proteins that circulate in the blood as inactive precursors. When activated in a cascade manner (by antibodies bound to antigen or directly by microbial surfaces), it provides immunity in the following ways:

(i) Lysis of pathogens: The activated complement proteins form a membrane-attack complex (MAC) which makes pores in the bacterial cell membrane, causing the cell to swell and burst (cytolysis). 

(ii) Opsonisation: Complement proteins coat the surface of the microbe and make it more easily recognised and engulfed by phagocytes (macrophages and neutrophils). 

(iii) Chemotaxis and inflammation: Complement fragments attract phagocytes to the site of infection and promote an inflammatory response, which increases blood flow and helps in the clearance of pathogens. 

(iv) Agglutination and neutralisation: They clump the pathogens and neutralise the viruses, making them easier to destroy. 

(v) Enhancement of antibody action: The complement system amplifies the effect of antibodies, thus strengthening both innate and acquired immunity.

The complement system, therefore, is a powerful amplification system that destroys pathogens directly and also helps phagocytes and antibodies to eliminate them.