Complement 3 (C3) is a central component of the complement system. It is a major opsonin, hence Complement 3 deficiency predisposes the patient to serious infections by pyogenic bacteria. Deficiency of C3 also results in autoimmune-like diseases. Specific therapy is not available, but vaccination for meningococcal and pneumococcal diseases is advised as a preventative measure.
The complement pathways are important parts of the immune system with roles in the fight against pathogens, inflammation, and autoimmune diseases. The complement system has three branches: the classical pathway is activated by interactions between antibodies and the early protein components of the classical branch; the alternative pathway is independent of antibodies; the third pathway is activated by lectins. All three branches result in the activation of complement 3 (C3) factor by proteolysis. Deposition of C3 products triggers the formation of the membrane attack complex (MAC) from the late components of the complement system. The membrane attack complex executes the killing function of the complement pathways. Deficiencies of the complement system could also lead to autoimmune diseases, such as systemic lupus erythematosus (SLE), and renal diseases.
Complement deficiency may be acquired, but in most cases is inherited. Deficiencies of complement components, particularly of C3, are rare . Defects of the early components (C1-C4) are often associated with autoimmune conditions , whereas shortages in late protein components lead to recurrent infections. Complement 3 is a central component of the pathways and a principal opsonin; its deficiency predisposes to infections with encapsulated bacteria, such as Neisseria meningitides. Autoimmune diseases are also present in a considerable proportion of patients who have decreased concentrations (usually less than 1% of the normal values) of C3.
History is an important part of evaluating the patient as recurrent infections are common, but specific physical signs are lacking in complement deficiencies. Recognition of pneumococcal and meningococcal infections is crucial. Recurrent otitis media and Bordetella pertussis pneumonia were reported in patients with complement 3 deficiency  . In one described case, an infant with Leiner’s disease, which is usually associated with C5 deficiency, was found to have a decreased level of C3 .
Early reports found evidence for recurrent infections in homozygous patients with C3 mutations  . Patients with complement 3 deficiency were also described with SLE-like conditions  . Some patients also developed glomerulonephritis. A more recent study, summarizing the phenotypic effects of C3 deficiencies based on 37 cases in 29 families, found that the overwhelming majority had recurrent infections, often with Neisseria meningitides, and that rheumatic and renal diseases each occurred in about a quarter of the patients . More than half of the patients with autoimmune diseases also had recurrent infections.
Laboratory examination of the hemolytic activity of the complement system is the cornerstone for testing the function of the classical complement pathway. One CH50 unit is the dilution or volume of serum that causes the hemolysis of 50% of sheep erythrocytes coated with antibodies . An analogous value can be determined for the alternative pathway. When both pathways are dysfunctional, it is likely that the C3 or other common components of the pathways are affected.
The levels of individual components of the complement cascades can be determined by using the enzyme-linked immunosorbent assay (ELISA) technique. A recent study using dried blood spot samples, routinely collected from newborns, and reverse phase protein microarrays successfully identified patients deficient in C3  . This approach is well suited for large-scale screening of neonates and has a potential for future use.