An 8-year-old boy suffers from frequent and severe bacterial infections. The fam

Question

An 8-year-old boy suffers from frequent and severe bacterial infections. The family physician thinks he suffers from agammaglobulinemia (absence of antibodies). The tests performed indicate he has normal immunoglobulin levels. However, in doing the blood work, a deficiency in complement protein C3 was discovered.

a. What is the role of C3 in the normal complement cascade?

b.Explain why a deficienceny in C3 would result in frequent bacteial infections experienced by the patient?

c. Would his symptoms be different if he were defecient for the C2 protein or C9 protein? If yes, how.

Explanation / Answer

Complement proteins are soluble serum proteins. The Complement cascade is a series of chemical reactions in which the products of one reaction become the catalysts for the next reaction. Complement proteins are a group of enzymes that are activated by several triggers. Complement proteins form a large lipophilic/hydrophobic membrane attack complex in the pathogen cell membrane and kill them.

i. An antigen-antibody complex (Classical pathway)
ii. Lectin coating the mannose, a carbohydrate found on the surface of fungi, bacteria, and viruses (Lectin pathway)
iii. C3b binding to the surface of a microbe (Alternative pathway).

Functions of complement system:
i. Inflammation: Complement proteins attract macrophages and neutrophils resulting in inflammation.
ii. Membrane attack: Complement proteins form a membrane attack complex (MAC) and lyse the pathogen cells.
iii. Phagocytosis: Complement proteins opsonize antigens.

C3 is an important factor of the complement system. C3b has the maximum opsonizing activity. ACtivation of C3 is required for both classical and alternate pathways. C3a is an anaphylatoxin and serves as the precursor for some cytokines.
People with C3 deficiency exhibit recurrent bacterial infections as C3 is involved in killing bacteria using MAC.
Mutations in C2 and C9 exhibit almost similar effect as C3 mutants. C2 is required for C3 activation and C9 is a component of the MAC.

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