system
Given the following post, respond as if you're commenting on Reddit:
human
If regularly donating blood without being re-exposed to a particular pathogen, will circulating antibody levels decline over time?

I’m not an immunologist so please forgive me if there are glaring errors. 

My understanding as I remember it from many years ago is that when a previously encountered pathogen is detected by B-memory cells, they proliferate and differentiate into plasma cells, which begin to produce antibodies until the pathogen is cleared. Whilst the B-memory cells will survive, the plasma cells and antibodies in the blood stream will decline over time until the pathogen is reintroduced and more are produced? 

I ask this in relation to Covid as I had Covid-19 back in early March and was very sick. However I’ve just had a negative antibody test when trying to donate plasma. I’ve donated blood on at least 4 occasions since and have been very careful with distancing etc so would hope I have not come back into contact with any viral particles.
gpt
Nah. Not really. You lose blood constantly, it just gets recycled internally and/or passed as waste. Antibodies themselves are just these little protein markers that act as cues to the rest of your immune system. They are produced constantly, quasi-randomly, but in low numbers. When they start "sticking" to things, your immune system takes notice. It obviously starts attacking the "flagged" intruders, but it also starts preferentially cloning the antibody (or rather the cell that produces that antibody). That's really why you get sick at all (at least, among illnesses you've had before). Your body may have the template "on file", but it takes a bit to ramp up production.