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

Leiner's disease, also known as erythroderma desquamativum, is a severe form of seborrheic dermatitis that typically begins in infancy. It is associated with recurrent infections and diarrhoea. The condition has been linked to deficiencies in the complement system, particularly C5. Complement protein C5 plays a crucial role in the formation of the Membrane Attack Complex (MAC), which assists in cell lysis during immune responses. A deficiency in this protein predisposes individuals to recurrent infections due to an impaired capacity of their immune system to effectively lyse bacterial cells.

C5-9 deficiency