C4 is a component of both the classical and alternative pathways of complement, and has a similar structure to that of C3. INCREASED C4 levels are found in association with C1 deficiency, acute phase reactions, and certain malignancies. DECREASED levels are associated with immune complex disease, SLE, autoimmune thyroditis, bacterial or viral meningitis, Streptococcus or Staphylococcus sepsis, and pneumonia. For more on the classical and alternative pathways, refer to C1q, First Component of Complement, & C3, Third Component of Complement, Serum.
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