Idster, antibodies work. Or, no one would recover.
How Long? Well, that is a valid question. They work at least till you recover from the disease…around 4-6 weeks. Are they immediately flushed out of your system? (I am guessing they have a half life similar to RBC – red blood cells – at 120 days. They are “known” to be human in origin.) Are they still being produced in response to “triggering” infections? (Those infections that could cause the disease with no antibodies present usually trigger the production of more antibodies. However, if this cell dies, then it does not.) This is a simple biochemical feed-back loop. Can it be extracted from a doners blood? This is the million dollar question, some proteins are too fragile for all but complicated extraction techniques. Since they have a test for this, I doubt it is too fragile, though.

