I put this here instead of the quarantine zone, because I’m asking generally about acquired immunity. My understanding has been that the immune system, when first stimulated takes some time to produce antibodies. But after the infection, the antibody cells stop proliferating, or at least stop making a lot of those antibodies…but those stimulated/activated cells are still present and tend to be available and ready to reproduce again upon a subsequent infection.
So, my understanding from what little immunology I learned a few decades ago is that it is expected that the antibody titer will fade over time, but that it is often (not always) expected that the ‘memory’ cells are there and ready to proliferate upon contact with the antigen and re-establish a large titer of antibodies, much more rapidly than the first contact.
Am I wrong in my memory? Isn’t it fairly common for antibody concentrations to be reduced over time, but that immunity to that same antigen is still present?
You have the essentials almost right, but I’ll expand on an important details using chicken pox as the example. Chicken pox is caused by the varicella zoster virus (VZV). The body produces two kinds of antibodies in response to the infection.
Early in the infection with VZV the body produces Immunoglobulin M (IgM) antibodies. IgM antibody levels increase during the course of the infection, plateau, and then decline. Even though the VZV virus is often not completely cleared (becoming dormant in certain nerve cells giving rise to possible later bouts of shingles) the IgM antibody levels generally fall below the level of detectability in a normal laboratory test.
But there is a second type of antibody produced. Immunoglobulin G (IgG) antibodies are produced later in the infection cycle, increase and then plateau. IgG antibodies are an important part of the long term protection from reinfection.
Even if IgG levels fall there are still memory cells in the immune system. Such a cell is a sort of biological blueprint for producing a particular antibody. Upon reinfection these memory cells divide and some of the daughter cells differentiate into plasma cells that excrete the relevant antibody.
It is important to understand that reinfection happens. Presence of the virus from such reinfection may be detectable in laboratory tests even if the patient has no symptoms. But since the body has already learned how to make the right antibody and has those memory cells in waiting the immunes system can react fast enough to generally prevent illness.
Adaptive immunity (which is what the OP is talking about) varies between individuals, and different pathogens induce variably effective immune responses.
"Across any population, there is a high degree of individual variability in our antibody responses to a pathogen in the amount, type and quality of antibodies that we make.
Some people make many high quality antibodies that are very good at recognizing the relevant antigen and binding to it. If this happens, the virus is rapidly bound by antibodies and eliminated before it can even cause an infection.
Other people make antibodies, but they’re not as effective at binding the pathogen. In this situation, the antibodies only provide partial protection: they slow the virus down but the virus can still cause some degree of infection. These individuals usually exhibit some symptoms and shed the virus for a longer period of time.
There are also some people who either produce very little or very poor quality antibodies. In this case, although these people produce antibodies, the immunity is not very effective so they can experience prolonged infection with more severe symptoms. They are also likely to be re-infected at a later point in time. This is one of the big unknowns with this new coronavirus: What percentage of the population falls into this category?"
Antibodies levels sometimes, but not always, fade, and there are memory B cells that can often quickly produce more on re-exposure, as well as memory T cells that can quickly mount a response that attacks infected cells without antibodies. Sometimes antibody levels stay high for a very long time, and the persistence of those levels have been assumed to be a good proxy of continued immunity. See this article from 2007 that documents extremely sustained antibody levels for several of the vaccine preventable viral diseases.
there is a good article on this subject in the NY Times today (sunday). I won’t bother linking since the NYT is paywalled, but if you can access the free version, it is a good article.
After the active infection is resolved, some of the antibody-producing B cells head off to a handy lymph node and hang out there forever, turning into memory B cells. Since nothing in biology is perfect, yes, even the memory B cells tend to gradually die off over time.