This novel coronavirus is new to the human population—we have never been exposed to it before—so there are many unknowns about how we respond to it. 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? Do antibodies always form after an infection?
We generally expect antibodies to form following infection, but there are certain cases where this might not occur. The adaptive immune system, which is what we have been talking about so far, is only one part of our immune response. We also have another type of immune system known as the innate immune system. The innate immune system is our frontline defense, the first system to respond to a new infection. This includes cells such as neutrophils, macrophages, and dendritic cells.
Unlike the adaptive immune system, which includes antigen-specific antibodies that take time to develop, the innate immune system responds to antigens very quickly but in a non-specific way. It attacks anything that “looks” foreign to the body, like components of a bacterial cell wall, or viral RNA and DNA. Quite often, the innate immune response will take care of an infection before the adaptive immune system even has a chance to start manufacturing antibodies.
The adaptive immune system involves more than just B cells, plasma cells, and antibodies—it also includes T cells. T cells are another population of white blood cells that can develop into memory cells, just as B cells can. They can also differentiate into specialized cells that kill virus-infected cells. The functions of T cells and B cells are different.
B cells develop into plasma cells that produce antibodies (T cells do not); T cells directly kill virus-infected cells (B cells do not). Sometimes individuals with a very vigorous T cell immune response will be protected from a pathogen even though they produce low amounts of antibody. The T cell immune response is much more difficult to measure than the antibody response and is usually only evaluated in specialized research settings.