The Antibody Test
The Covid-19 antibody test uses blood from a pin prick whereas the antigen test uses material gathered by a nasal swab.
With the antibody test one looks for 2 types of antibodies, IGM and ICG. IGM is very active within the first day of infection. So if you're have IGM, we know that you have a current ongoing infection.
The presence of ICG antibodies indicates that you have essentially a very strong immunity to the corona virus. This usually appears about two weeks after you were infected. This signals that your body is fighting back with a stronger form of an antibody.
The Covid-19 Antigen Test
With the antigen test, using a swab, one is looking for proteins created by the coronavirus. However this antigen, is very similar to many different types of other virus proteins. So there's a lot of false positives that tend to happen with this type of Covid-19 test. On the other hand, testing for antibodies the detection is very specific, very sensitive and more accurate. So that's the real distinction between the two types of tests.