July 01, 2010

MRCP 1 Question 9

A patient with recurrent infections with encapsulated bacteria is found to have a primary complement deficiency.

Which two primary complement deficiencies most commonly predispose to infection by encapsulated bacteria?

a) C1 and C2
b) C4 and C9
c) C3 and C5
d) C6 and C7
e) C8 and C9

The correct answer is C


The complement system consists of a group of circulating proteins that have immunologic effects when they are activated.

Complement activation serves first to opsonize pathogens for phagocytosis; second, to attract inflammatory cells; and third, to kill the pathogen by creating pores in its membrane.

Because these functions are closely tied to the actions of antibodies, defects in the complement system result in recurrent infections with extracellular bacteria, including encapsulated bacteria.

Primary deficiencies of C3 or C5 predispose the host to infection by encapsulated bacteria . Deficiencies of the later complement components (ie, C5b, C6, C7, C8, and C9) cause vulnerability principally to Neisseria gonorrhoeae and Neisseria meningitidis. Secondary (ie, acquired) complement defects are seldom clinically important.

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