Complement component 4 (C4) is a protein involved in the complement system, originating from the human leukocyte antigen (HLA) system and serves a number of critical functions in immunity, tolerance, and autoimmunity with the other numerous components.
The complement system can be activated via the classical and the alternative route. Complement factor C4 participates in activation by the classical route. A decrease in C4 is common, but complete absence is rare. A lowered concentration or the complete absence of C4 occurs in immunocomplex diseases, systemic lupus erythematosus (SLE), autoimmune thyroiditis and juvenile dermatomyositis.
Infections such as bacterial and viral meningitis, streptococcal and staphylococcal sepsis and pneumonia are associated with a fall in C4. As an acute phase protein, C4 is produced to an increased extent during inflammatory processes.
It is elevated in systemic infections, noninfectious chronic inflammatory conditions (primarily chronic polyarthritis) and physiological states (pregnancy).The elevation rarely exceeds twice the normal value and can mask a reduction in the current consumption.
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