ELISA test

ELISA is the name of a laboratory method, not of a disease or a result. It is used to detect and measure immune reactions, that is, encounters between an antigen, the substance the body recognizes as foreign, and an antibody, the protein that binds to it.

It all happens inside a well, a small plastic dimple in a plate. The trick is that one of the two players is stuck to the bottom. Because it is anchored, everything else can be washed away: whatever did not bind leaves with the wash, and whatever did bind stays behind. In the most widely used version, the antigen is trapped between two antibodies, and the second one carries an enzyme hanging off it. The enzyme chooses nothing: it converts a substance and generates a signal, and the amount of that signal is proportional to how much antigen was in the sample.

If what you are after is the antibodies rather than the antigen, the setup is reversed: the antigen is stuck to the bottom and you see whether the antibodies in the sample latch onto it. That format is called an indirect ELISA, and it is the one usually used to detect antibodies against a virus.