@thor they look for anti_gens_ using anti_bodies_. i can't find a really good explanation now, but this quote may explain it:

> This SARS-CoV-2 Antigen Rapid Test Kit (Colloidal Gold) uses the sandwich immunocapture method and colloidal gold immunochromatography to qualitatively determine the presence of SARS-CoV-2 antigens. During the test, the sample is dropped into the sample well, and SARS-CoV-2 antigens in the sample are bound by colloidal gold-labeled monoclonal anti-SARS-CoV-2 antibodies. These then migrate together to the test area, as the capillary effect draws the fluid up the nitrocellulose test strip. A row of SARS-CoV-2 monoclonal antibodies are coated on the strip at the test area (T line), and these also bind the antigens. The antibody-antigen-antibody-colloidal gold sandwich results in a visible line at the T line, if the SARS-CoV-2 antigens are present in the sample. The quality control area (C line) is coated with goat anti-mouse antibodies that bind the colloidal gold-labeled antibodies, resulting in a visible C line. If the C line does not show any colour, it indicates that the result is invalid, and this patient needs to be tested again.

(from https://www.antibodies.com/sars-cov-2-antigen-rapid-test-kit-colloidal-gold-a254381)