Modern ELISA for determination of functional activity of component С2 and factors B and D, proteinases of a complement system, and component C3, substrate C3-convertases, key complex enzymes of the complement, have been developed. Essential feature of С3-convertases classical (C4bC2a) and alternative (C3bBb) pathways of the complement activation is that their substrate С3 after proteolytic cleavage is converted into C3b, carrying on the surface thioester covalent bond linking C3b with nucleophilic acceptors that results in immobilization of this proteolytic product near the activating enzyme.
Cascade character of an activation of complement system allows to create artificial deficit of separate components in the experimental system and to determine (by ELISA) covalently immobilized component C3 during activation, and also to determine functional activity of any of pre-exhausted components. Use of such approach resulted in the development of the ELISA systems suitable for determination of functional activity of component С2 of classical pathway and factors B and D of the alternative pathway by testing quantity of the immobilized C3b at excess C3.
The developed methods allow to investigate mechanisms of functioning of complement, inhibition of the cascade activation by endogenic and exogenous inhibitors, and also to find functional deficiency of components in serum and other biological fluids.