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Given the surge in interest by the media in humoral (antibody) immunity - I felt it would be nice to give a brief overview of how it works in laymen terms... since I have almost no followers and am not a B cell expert I welcome sharing/addendums if anyone cares to do so..
Immunity to pathogens (like SARS-CoV2) comes in various flavors but long-lived recall (memory) immunity is typically associated with both T and B lymphocyte responses. T cells function by killing infected cells and providing help to other cells of the immune system-incl. B cells
B cells generally are thought to work by producing antibodies which bind to pathogens and prevent them from infecting new cells and help the body get rid of them.
In the context of vaccines and 'herd immunity' the general focus is on B cells, and in particular on one type of B cell which is called a Plasma Cell. These cells constantly produce antibodies (Abs) which are measured in seropositivity tests in blood.
However, other B cells also are generated after vaccines/infections called memory B cells. These don't produce lots of Abs in the absence of an infection. But if a person is re-infected they can rapidly produce tons of new plasma cells.
A quirk of B cells is that each time a person is re-exposed the memory B cells can evolve to make better Plasma Cells with more specific Abs.
Historically for some reason we have viewed vaccines success based on how long circulating antibodies get produced after vaccination. In the case of the most famous vaccines (measles, mumps, rubella, polio, smallpox) Plasma cells can live a LONG time.
The most famous study I know of - Amanna et al NEJM 2007 - found that these vaccines gave rise to Abs that stick around for a human lifespan. Whereas other vaccines like Tetanus were shorter (10-16 years)
Although vaccines are typically re-administered if long-lived Ab responses are not seen - it is actually REALLY hard to find out whether one is still immune to a pathogen. That is because memory T and B cells are likely still there even in the absence of detectable Abs!
So why do some vaccines give rise to lifelong titres of circulating Ab? This is to my knowledge completely unknown. But it has historically been linked to the longevity of the Plasma Cells generated during the active immune response
This was first investigated closely in laboratory mice where Plasma Cells were shown to be most abundant in the bone marrow and could be tracked. Here they found that plasma cells lived a long time - leading to the widespread belief that plasma cells are long-lived.
But there are two catches! (1) a long time in a lab mouse turns out to be not so long in human year - more like a few hundred days and (2) these mice live in pathogen free environments so they are really only ever infected or vaccinated by a single thing.
Some colleagues of mine recently used a method from our lab (14-Carbon measurement in cell DNA) to assess how long Plasma cells live in a human. They found that in the intestine there are lots of Plasma cells and some can live a long time - perhaps as long as a person
So perhaps some plasma cells can pump out antibodies for your whole lifetime. However plasma cells in the intestine probably make Abs specific for intestinal pathogens or even your own friendly bacteria. For SARS you probably want Plasma Cells producing Abs in the blood.
These may be resident in the peripheral organs or in the bone marrow and it is unclear how long they can live for and why some persist and others don't.
Nonetheless - there is absolutely no reason to assume that if antibody titres decrease after the clearance of an infection this means a person is not protected from subsequent infections! There is already evidence that T cell immunity persists a long time for SARS-1
And good evidence for T cell memory developing against SARS-CoV2. It is very likely B cell memory exists for SARS as well. So we should all be a bit more critical when we wonder what protective immunity looks like - and not forget that it comes in MANY more forms than just Abs.
