C3 deficiency --> Type II hypersensitivity?

[catzzz88]

(1) Can someone explain why C3 deficiency can lead to increased susceptibility to type III hypersensitivity reactions (p. 199 FA 2016)?

• I get that C3 deficiency leads to lack of C3b, so less opsonization, so higher incidence of infection with encapsulated bacteria.
• However, I am missing the link to hypersensitivity reactions, please help!

(2) Also, anyone know why C5-C9 deficiency increases risk of Neisseria, specifically? Like, why not other bacteria if you cant for the MAC complex?

Best,
C

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[Adjet]

As far as Q1: "Opsonins—C3b and IgG are the two 1° opsonins in bacterial defense; enhance phagocytosis. C3b also helps clear immune complexes." from p. 199 in FA 2016. A type III hypersensitivity reaction involves the deposition of immune complexes so decreased IC clearance would make one more susceptible to a hypersensitivity reaction

Don't know the answer to #2, but my guess is that the immune response to Neisseria is particularly dependent on the MAC compared to other microbes. Sorta just memorizing that one though, since it's straightforward enough