C1 inhibitor (C1-INH) protein deficiency

• causes hereditary angioedema
• C1-INH is a multifunctional serine protease inhibitor
• probable mechanism is uncontrolled release of bradykinin resulting in oedema of tissues
• low C2 and C4 d/t uncontrolled complement activation and comsumption
• Danazol can raise the level sufficiently to prevent attacks
• 15% may have normal level but functional defect

C1q, C1rs, C2, C4 deficiency (classical pathway components)

• predisposes to immune complex diseases
• e.g. SLE, Henoch-Schonlein Purpura
• also predisposes to bacterial infections, but less severe than C3 deficiency

C3 deficiency

• presents early in life; can be fatal without treatment
• causes recurrent bacterial infections esp. encapsulated organisms (eg. pneumococci)

C5 deficiency

• predisposes to Leiner disease
• recurrent diarrhoea, wasting and seborrhoeic dermatitis

C5-9 deficiency

• encodes the membrane attack complex (MAC)