What is Complement C4 (Complement Component 4)?

What is Complement C4?

Complement C4 is a central protein in the classical and lectin pathways of the complement system—a key component of innate immunity that enables immediate recognition and elimination of invading microbes, immune complexes, and cellular debris. 1

Basic Structure and Function

Complement C4 is a non-enzymatic component essential for propagation of the classical complement pathway. 2 The protein functions through several key mechanisms:

• C4 becomes covalently attached to pathogen surfaces and altered self-tissue after activation, where it acts as an opsonin marking targets for removal 3
• C4b (the major activated fragment) provides a platform for assembly of proteolytically active convertases (C3 and C5 convertases) that mediate downstream complement activation 3
• C4 enhances solubilization of immune aggregates and clearance of immune complexes through complement receptor one (CR1) on erythrocytes 2

Genetic Complexity and Polymorphism

C4 is the most polymorphic protein in the complement system, with remarkable genetic diversity. 1, 2

Gene Structure

• C4 genes are located in the MHC class III region on chromosome 6, with each gene consisting of 41 exons coding for a 5.4-kb transcript 2
• Two main forms exist: C4A and C4B, which differ in only four amino acid residues but have dramatically different binding properties 4
• Gene size varies: long genes are 20.6 kb and short genes are 14.2 kb, with three-quarters of C4 genes harboring a 6.4 kb endogenous retrovirus (HERV-K) in intron 9 2

Copy Number Variation

• In Caucasian populations, 55% have 2-locus C4A-C4B configurations and 45% have unequal numbers of C4A and C4B genes 2
• Each composite gene varies between 0-5 copies per haplotype 5
• Partial deficiencies of C4A and C4B have a combined frequency of 31.6% in the population 2

Functional Differences Between C4A and C4B

C4A and C4B exhibit dramatically different binding specificities despite minimal sequence differences:

• C4A binds preferentially to protein antigens through amide bond formation, with residue D1106 responsible for effective binding to immune aggregates 2
• C4B binds twice as effectively to carbohydrate antigens through ester bond formation, with residue H1106 catalyzing transacylation reactions with hydroxyl groups 2, 4
• This functional divergence provides biological advantage in reacting with a wide range of antigenic structures 4

Clinical Significance

Role in Disease Diagnosis

C4 and its degradation products (particularly C4d) serve as critical diagnostic markers:

• C4d is a complement split product that binds covalently to endothelium at sites of complement activation and persists longer than C3 or C1q 6
• In transplant rejection, C4d deposition is largely restricted to capillaries and serves as an immunopathologic marker for antibody-mediated rejection 6
• Combined C4d and C3d positivity predicts graft dysfunction and mortality better than C4d alone in cardiac transplantation 7

Baseline Laboratory Assessment

C4 levels should be measured as part of baseline complement assessment in several clinical contexts:

• In systemic lupus erythematosus (SLE), baseline C4 and C3 should be measured along with autoantibodies (ANA, anti-dsDNA, anti-Ro, anti-La, anti-RNP, anti-Sm, anti-phospholipid) 6
• Serial C4 and C3 measurements may support evidence of disease activity or remission in SLE patients 6
• In glomerulonephritis with MPGN pattern, C3/C4 levels help distinguish between immune complex-mediated disease, complement-dominant disease, and immunofluorescence-negative disease 6

Association with Infections and Autoimmunity

C4 deficiency creates susceptibility to both infections and autoimmune disorders:

• Individuals with C4 deficiency are prone to microbial infections due to impaired complement-mediated clearance 1
• C4 deficiency links to autoimmune disorders, with the correlation between different C4 forms and autoimmune disease susceptibility potentially explained by differences in binding reactivity 4
• Patients with early complement component deficiencies (C1, C2, C4) can receive all viral vaccines but require special emphasis on pneumococcal and meningococcal vaccines 6

Expression and Regulation

• The liver is the main organ synthesizing and secreting C4A and C4B to circulation, though many extra-hepatic sites produce moderate quantities for local defense 2
• Plasma protein levels of C4A and C4B are mainly determined by gene dosage, though C4B proteins from short genes may have relatively higher concentrations than those from long C4A genes 2
• C4 expression is inducible or enhanced by gamma-interferon 2

Common Pitfalls in C4 Assessment

When interpreting C4 levels and C4d staining, several caveats apply:

• Only capillary staining with C4d is considered significant in transplant biopsies; staining of large-vessel endothelium, perimyocytes, or sarcolemma does not indicate antibody-mediated rejection 6, 7
• C4d positivity alone is insufficient for AMR diagnosis—it should be combined with histological features, donor-specific antibodies, or clinical graft dysfunction 6
• C4 levels may decrease transiently after vaccination in some autoimmune patients (particularly C3 and C4 at day 13 post-vaccination), though values typically remain in normal range 6