A 2-year-old girl is brought to the emergency department by her mother due to fever for the past day and acute-onset seizures in the last hour. Past medical history is unremarkable, and immunizations are up to date. Her temperature is 38.7°C (101.6°F), pulse is 125/min, respirations are 26/min, and blood pressure is 90/55 mm Hg. Physical examination shows mild tachycardia, tachypnea, and a non-blanchable purpuric rash on the trunk and lower extremities.
Deficiency in which of the following complement proteins increases the risk for recurrent infection with the suspected etiologic agent in this case?
E) C5b
The patient presents with fever, seizures, and purpuric lesions suggestive of a meningococcal infection. Neiserria meningitidis is a gram-negative diplococcus and is the causative agent of meningococcal infections. Complements C5b-9 make up the terminal complement complex, also known as the membrane attack complex (MAC). MAC inserts into N. meningitidis’ cell membrane and causes lysis of the meningococci. Deficiency in complements that form MAC causes recurrent infections from Neisseria, as seen in this patient.
Answer choice A: C3a, is incorrect. C3A is an anaphylatoxin that causes histamine release from mast cells, which leads to vascular permeability, smooth muscle contraction and urticaria.
Answer choice B: C3b, is incorrect. C3b functions as an opsonin in the complement pathway. It binds to a pathogen and allows it to be targeted by phagocytes that have C3b receptors. C3b is not a part of the terminal complement complex.
Answer choice C: C4a, is incorrect. C4a also is an anaphylatoxin that causes histamine release from mast cells, which leads to vascular permeability, smooth muscle contraction and urticaria.
Answer choice D: C5a, is incorrect. C5a is an anaphylatoxin that causes histamine release from mast cells, which leads to vascular permeability, smooth muscle contraction and urticaria.
Key Learning Point
Deficiency in terminal complement proteins C5b-9 increases susceptibility to N. meningitidis infections.
