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Two isotypes of human C4, C4A and C4B have different structure and function.
1Département de Médecine, Hôpital Cantonal Universitaire, Geneva, Switzerland.
Complement and Inflammation
|January 1, 1989
Summary
Human complement component 4 (C4) exists as two types, C4A and C4B, with distinct functions. C4A preferentially binds immune complexes, crucial for clearing pathogens and preventing autoimmune diseases like lupus.
Area of Science:
- Immunology
- Complement System Biology
- Biochemistry
Background:
- Human complement component 4 (C4) comprises two main isotypes, C4A and C4B.
- These isotypes exhibit distinct biochemical properties and functional roles within the complement cascade.
Purpose of the Study:
- To elucidate the functional differences between C4A and C4B, particularly in substrate binding and immune complex processing.
- To explore the implications of these functional differences in the context of host defense and autoimmune disease pathogenesis.
Main Methods:
- Comparative analysis of C4A and C4B activity in hemolytic assays.
- Assessment of C4A and C4B binding efficiency to immune complexes.
- Evaluation of C4-mediated C3 convertase formation on various substrates.
Main Results:
- C4B demonstrates higher efficiency in hemolytic assays compared to C4A.
- C4A exhibits preferential binding to immune complexes, with a greater than fivefold difference observed in assays comparing hemolysis and immune complex processing.
- The classical complement pathway functions as a duplicated system, enabling C3 convertase formation on diverse substrates.
Conclusions:
- The functional dichotomy between C4A and C4B provides a robust mechanism for eliminating invading microorganisms.
- Defective immune complex processing due to C4A deficiency may contribute to the increased incidence of systemic lupus erythematosus in individuals with homozygous C4A null alleles.