ABSTRACT
The complement system is composed of 30 or more chemically and immunologically distinct proteins capable of interacting with each other, antibodies, certain bacterial products, and cell membranes. In most studies performed to evaluate C4 in various animals, internal laboratory standards rather than commercially available reference standards were used; therefore, a direct comparison between laboratories is often difficult. This chapter presents a more complete discussion of the various methods and reviews the technical aspects of the hemolytic assays for various domestic species. In human beings, C4 acts as an acute phase reactant with levels increasing two- to threefold during acute inflammation. Elevations in C4 are also parallel increased corticosteroids. In human beings infected with malaria, there is rapid activation of the classic pathway and a resultant sharp decline in serum C1, C4, C2, and C3 levels concomitant with schizont rupture and intravascular release of malaria antigens.
