C4 is a protein belonging to the compliment system of proteins. It is one of the
9 major proteins in the compliment system (C 1 TO C9). The compliment
system proteins play a major role in the immunity of a person. The levels of the
protein are measured to estimate the activity of an immune disease especially
in autoimmunity. The levels will decrease in case of an active disease indicating
that the compliment has been used up and consumed rapidly. The levels of C4
may increase in response to acute or chronic inflammation and certain
malignancies.