system
Given the following post, respond as if you're commenting on Reddit:
human
Do antibodies get used up when preventing an infection?

Suppose I have COVID-19 neutralizing antibodies from a vaccine. I get exposed to SARS-CoV-2, but neutralizing antibodies prevent the virus from reaching my cells. Does that lower my antibody levels, leaving me more vulnerable on next exposure? Would antibody levels drop for other reasons than this? Can the distinction (if this depletion does occur) be measured?
gpt
Yes, antibodies get used up while preventing an infection. 

Antibodies also get used up when *not* preventing an infection, and the difference in rates is pretty small. 

Antibodies normally only have a half-life of 2-3 weeks. The reason antibodies seem to remain roughly constant is that they’re being continuously made, at roughly the same rate they turn over. They’re constantly being made by plasma cells, which are renewed by memory B cells (I am simplifying wildly here). 

If there’s an infection, antibodies do get used up as they bind to pathogens, which might get destroyed by white blood cells or simply blown out your nose, covered with antibodies. So the rate of destruction of antibodies during an infection increases. 

But B cells also respond to the presence of the pathogen by making more plasma cells and more antibodies. A single plasma cell can make thousands of antibody molecules every second, and there may be millions of plasma cells during an infection; overall, infection leads to a dramatic increase in antibody concentration even though some are being used up while attacking the pathogen.