C3 defic. and C5-C9 defic.

[LuckiestOne]

C3 defic. leads to lower resistance against capsulated bacteria, is this because of lack of opsonization?

Why does C5-C9 deficiency only related to Neisseria? is that the only organism that is mostly destroyed by MAC?

Wouldn't C3 deficiency also cause Neisseria resistance weakening by not allowing the steps before C5-C9 happen?

Thanks in advance!

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[Phloston]

C3 defic. leads to lower resistance against capsulated bacteria, is this because of lack of opsonization?

C3b and IgG are the two major opsonins (says this in FA somewhere). Therefore, lack of C3 (and that is, C3b) leads to susceptibility to encapsulated organisms. This is also another reason why hypo- or asplenia results in similar susceptibility. Since the spleen is such a substantial secondary lymphoid organ, its removal means decreased antibody production and opsonization ability (in addition to the fact that ~50% of the body's reservoir of cells of monocytic lineage are lost).

Why does C5-C9 deficiency only related to Neisseria? is that the only organism that is mostly destroyed by MAC?

That's always been my understanding of it. I wouldn't even go so far to say that that's the only organism as much as I would to say that it's probably just the most clinically relevant one that the USMLE wants us to know.

Wouldn't C3 deficiency also cause Neisseria resistance weakening by not allowing the steps before C5-C9 happen?

Thanks in advance!

If you look at the diagram on p. 229 of FA2012, it appears as though the MAC complex can still be assembled independent of C3 insofar as the lectin and classical complement pathways are still functional. We wouldn't really be dealing with Ag-Ab here (classical), but the lectin could still work to generate MAC in the event of Neisseria. I could be completely wrong here, but this very well might have just turned into a good USMLE question: Which of the following complement proteins are necessary to knock out Neisseria in a pt with C3-deficiency? --> anything in the lectin pathway.

 

[OveractiveBrain]

C3 defic. leads to lower resistance against capsulated bacteria, is this because of lack of opsonization?

Why does C5-C9 deficiency only related to Neisseria? is that the only organism that is mostly destroyed by MAC?

Wouldn't C3 deficiency also cause Neisseria resistance weakening by not allowing the steps before C5-C9 happen?

Thanks in advance!

Great explanation above. Honestly, memorize this and then forget it. its worth ONE question on Step 1.