The Complement System

The Complement System:

The serum of the blood contains a large number (over 30) of serum proteins that circulate in an inactive state and following their initial activation by specific (adaptive) and nonspecific (innate) immunogenic mechanisms, interact in a highly regulated cascade-fashion in which the activation of one component results in the activation of next in the cascade. This cascade of scrum proteins is collectively called the complement system and the serum protein of the complement system are called complement proteins.

When the inactive forms of complement proteins are converted into active forms by various specific (adaptive) and nonspecific (innate) immunologic mechanisms, they damage the membranes of microbial pathogens either destroying them or facilitating their clearance.

Complement system may act as an effector system that is triggered by binding if antibodies to certain cell surfaces, or it may be activated by reactions between complement proteins and receptors of microbial cell walls. Reactions between complement proteins and cellular receptors trigger activation of cells of the innate or adaptive immunity.

There are three pathways of complement activation:

(i) Classical complement pathway,

(ii) Alternate complement pathway, and

(iii) Lectin complement pathway.

Although these pathways employ similar mechanisms, specific proteins are unique to the first part of each pathway. Classical pathway is involved in specific or acquire (adaptive) immunity, whereas both the alternate and lectin pathways play important role in innate (nonspecific) immunity.