Complement

• is consumed during active immune complex deposition
• SLE: in active disease, C3, C4, and CH50 are decreased
• CH50: "total hemolytic complement activity," tests the capacity of proteins in the classic pathway and membrane attack complex to lyse antibody coated RBC's
• AH50: measures the alternative pathway
• Decreased C4 means classic pathway activation
• Decreased C4 could mean activation of either pathway.

Complement deficiency

What portion of complement pathway is infected if presentation is Autoimmune, Neisseria infxn, or Angioedema? Answers below.

Initial complement deficiencies

• C1, C4, C2, C3
• Presentation: autoimmune syndromes
  • SLE, PAN, HSP

Terminal Complement deficiency

• C5-C8 (MAC complex)
• Susceptible to recurrent Neisseria infection
• >50% with C5-C8 deficiency have had meningococcal meningitis or extragenital gonococcal infection
• 10-15% with systemic meningococcal disease will have a C5-C8 deficiency

Presentation > Deficiency

1. Autoimmune > Initial complement
2. Neisseria infection > terminal complement
3. Angioedema > C1 Esterase Inhibitor

CHLA board review 2005