For determining whether or not a person has developed antibodies against the new coronavirus, a typical test would work like this: drops of blood are absorbed onto a “sample pad” at one end of the test device. That soaks up red blood cells and the like and lets the plasma soak along a laminate of what’s essentially paper. The first thing it encounters is a zone that has known coronavirus antigens (such as pieces of the spike proteins, etc.), which pieces are also linked, in the most common form, to tiny particles of colloidal gold metal. If the plasma has antibodies to the coronavirus proteins, those will bind to the test antigens and carry them (and their colloidal gold particles) along up the strip. Then it runs into three zones on the paper, narrow strips that are impregnated with “antibodies to antibodies” (yep, that’s a real thing).
I didn’t go into the various subclasses of antibodies in my quick explanation above (and yes, by immunology standards that was about as short as it gets!) But the tests are looking for two antibody subclasses, IgG and IgM. The IgM ones are the first that get produced in an immune response, mostly coming from the spleen, but they’re also relatively short-lived, with a half-life of five or six days. So detection of IgM against coronavirus antigens indicates a recent (or still active) infection. The IgG antibodies are more numerous in the end, though, and for many infections (measles, chickenpox, mumps, hepatitis B and more) they indicate that a person is now immune to re-infection.
So on that paper strip, the plasma will hit a band of anti-IgM antibodies, bound to the paper, and then a band of anti-IgG antibodies, and finally a band of control antibodies that react with human antibodies in general. Remember, the plasma is carrying the test patient’s antibodies that are holding onto antigens with colloidal gold particles tied to them. When these hit one of those antibody-to-antibodies zones, they’ll come to a halt there, and the colloidal gold particles will pile up enough in that zone to show you a red-pink color. So the test strip can show red lines for either IgG or IgM, both, or neither, but if there’s no red line in the control strip then something has gone wrong and the test needs to be discarded and run again with a fresh kit.
You can realize, then, that if a person shows positive for IgM only then they may well be actively infected. And if they show only IgG, they may well have gone through an infection and could be immune (more about that in a minute). Showing both, well, you’re probably on the back end of an infection? And showing neither (but with a valid control line) could mean that you haven’t been exposed to the virus at all. But wait! There are complications, because there are always complications with the immune response. It should be mentioned that if a person was infected with SARS a few years ago that they would also probably show positive in this test; I don’t think they are specific enough to distinguish although I’d be interested to hear more details. On the other side, a negative result really doesn’t mean much, because there’s always the chance that a person generated antibodies that don’t recognize the antigens that the test kit has built into it for detection. You can’t rule it out. It is also quite possible that a person has been infected but hasn’t had time to generate enough antibodies for the test to detect yet. All such kits will include a warning that negative result can’t be used to say that a person isn’t/hasn’t been infected. And they’ll also include a warning that such a kit can’t be used as the last word even if they come out positive, although to be sure it is a pretty strong indicator. And obviously, you’re not getting any information about the actual levels of antibodies (past “enough to show a red line” anyway) or how those levels might be changing.
A big question is whether this coronavirus infection will provide lasting immunity: is a person who has “seroconverted” and shows IgG against coronavirus antigens safe to go out without fear of re-infection? And that we don’t quite know yet. The record with past coronavirus pathogens is mixed. We’re going to know eventually, and it could be a key to get past this whole epidemic, but we need more data to be sure. We also don’t know how long such immunity will last, obviously. Months? Years? How many? There’s no way to speed that data collection up; we’ll find out as time goes on. An example is that many vaccinated people my age are still immune to rubella but not to measles. I had myself checked last year because of the increase in cases in the US and got re-vaccinated because I found out that my measles immunity had vanished. (Mumps I became immune to the hard way in about 1967!)