So I have been reading some preprint articles about how the immune system works to respond to the virus and have come up with a rough theory of what is going on. Here is one of the articles.
https://www.medrxiv.org/content/10.1....full.pdf+html
Anyways, it appears that early in the disease it is the T cells and NK cells that are fighting the virus. Which makes sense, as these are the major players in cell mediated immunity, including against viruses. It appears that for the majority of people the T cells and NK cells do their job and the virus clears.
For people that advance to serious disease though, their T cell count and NK cell counts plummet, especially the CD8 T cells that are responsible for killing virally infected cells. Looking at cellular markers such as PD-1, CCR7 and CD45 it appears what is going on is these cells are becoming exhausted and burned out, and the body is unable to replenish them. At the point the T cells start to burn out and T cells number start to drop: viremia, inflammation and antibodies all start to shoot up, and it is bad news. Additionally when T cells get highly differentiated and exhausted they start pumping out IFNy, which is an extremely potent inflammatory mediator, so exhausted T cells may actually be making the problem worse by contributing towards runaway inflammation and ultimately cytokine storm and sepsis.
There are a couple takeaways from this. T cells produce memory immunity in a similar fashion to plasma B cells (the cells that produce antibody). The main reason we measure antibodies as far as immunity is because they are longer lived than T cells and much much much easier to measure, not because they are more important. It is very possible that persons with low antibody counts still have some level of built up memory immunity, through T cell immunity.
Another interesting takeaway that supports the idea that T cells are very important in knocking out the virus early, is that T cells are made in the thymus. This is an organ behind the heart that is very big when you are a child but dies throughout your lifetime. So this could explain why this disease is handled so much better in children and gets worse and worse as you get older. Because they have much bigger, better thymus and can replenish T cells better.