C4 Deficiency

**Semantic type:** Disease or Syndrome

**Definition:** Lack of production of functional C4 protein, due to a genetic defect requiring homozygous loss of both the C4A and C4B genetic paralogs. Approximately 75% of patients with a C4 deficiency will develop a severe systemic lupus erythematosus at an early age. Patients also present with frequent sinopulmonary infections often with Streptococcus pneumoniae.