I am not typing in to the void!
Technical correction, antibodies are part of the immune system. Will do a very short primer.
Innate Immunity. Things you have that work against any pathogen. Neutrophils, stomach acid, lysozyme in tears, ciliary action in lungs... etc. etc.
Adaptive immunity: The part of the immune system that adapts and responds to pathogens and improves with time.
This can be broken down into cellular immunity and humoral immunity.
Humoral immunity: Antibody production. This is done by B-cells. Given the correct cytokine stimulation you can get memory B-cells or plasma cells. both crank out antibodies, the plasma cells probably giving the highest amount of antibody production. Memory B cells stick around so at a later date you can make antibodies again in the event of another challenge. (they don't get lost to lack of use by a booster) Booster also keeps circulating antibodies high so immune system is ready to go.
Cellular immunity: Here, T-Cells, Natural Killer cells. These cells have been adapted specifically to recognize virus infected cells or bad cells (cancer) in the body. So when you have an infection going on you are able to wipe out the actual infected cell and not let the virus spread.
This is a huge generalization of what is known and incomplete retelling. But when you get an antibody test you are only looking at one aspect of viral resistance, and that is circulating antibody. Testing for the t-cells/NK cells and even the memory B-cells is all significantly more difficult.
Side note: From the get go I have been #TeamModerna due to the fact they had been in the game longer and I know they had to be working on things to optimize the cellular immunity portion of the response. I say this, as a lot of their goal was/is a cancer vaccine, which is infinitely more difficult and really needs to involve the cellular immunity component of the host response.