Is/are a group of proteins that normally circulate(s) in the blood in an inactive form, and when needed, assist(s) antibodies to ward off pathogens by combining with them to dissolve and remove pathogenic bacteria and other foreign cells.

complement system

Explanation:

The complement system is one of the second line of defense. Complement system refers to the antimicrobial substances that are actually in a group of inactive proteins present in blood plasma. These proteins are also present on the plasma membrane. Activation of proteins of the complement system serves to enhance immune response, triggers bursting of pathogens, stimulates phagocytosis. These proteins are also involved in inflammation and prevent excess damage to the tissues of the body.

The inactive proteins of the complement system are denoted as C1-C9. The enzymatic splitting of these inactive proteins into active fragments triggers several immune responses. Binding of antibodies to antigen and formation of antibody-antigen complete activates C1 proteins of the complement system. This is followed by activation of C3 which in turn is involved in the initiation of phagocytosis, inflammation, etc.