Specific inactivation of the fourth complement component. II. In vitro studies.

The fourth complement component (C4) inactivator obtained from nurse shark serum was used to inactivate C4 in fresh sera of various mammalian species. These hemolytically inactive sera, which contained the remaining eight complement components in unaltered form, were used (i) as a source of the first complement component (C1) for the generation of sensitized sheep erythrocytes (EA)-C1 and (ii) as a homologous reagent for the measurement of C4 activity with EA-C1 or EA in fresh, normal serum of guinea pigs, humans, dogs, and pigs.