Let’s talk antibodies. Mounting an antibody response is crucial for anyone to overcome a challenge from an infectious pathogen, and the immunity that can result is crucial for entire communities and populations. Determining who has such immunity is furthermore going to be crucial for us as we come out from under the current pandemic. If someone is truly immune to re-infection (and if the infectious agent isn’t mutating out of range) then they should be able to freely leave their homes, go to work, encounter groups of people and life that normal life that is such a desirable state these days.
Here’s some immunology background for those who could use a refresher. As always, you can assume that for every detail mentioned here that there are further details and complications behind them – that’s always true of human biology, but it is never more true than when you’re talking about the immune system. First off, there’s a big division into the innate immune system and the adaptive immune system. The first one is evolutionarily older, and is the general-purpose always-on component. I’m going to give it very little detail here in the interest of space, and because it’s adaptive immunity that is the key issue in the current outbreak, but it involves the familiar and extremely important processes of inflammation.
The innate immune system relies on general “that ain’t right” signals, such as the ability of toll-like receptors (TLRs) to recognize unusual double-stranded RNA floating around as a sign of viral infection. The RIG-I receptor family also recognizes several viral RNA motifs – viral replication is necessarily going to involve lots of production of such species, so a constant . Naturally, over the eons viruses have also evolved methods of their own to disrupt these recognition signals and their downstream events (many mediated by type I interferons). Another sign of viral infection is cells that have deficiencies in their major histocompatibility complex (MHC) surface proteins. The MHC is famously the body’s “identification-friend-or-foe” (IFF) system for recognizing “self” versus “nonself”, and is the basis for things like transplant rejection. NK (natural killer) cells are constantly patrolling for cells that are displaying “insufficient self” and destroying them, and that can mean cells whose functions have been disrupted by viral replication going on inside them.
These are all valuable functions, but there are many situations where they won’t be enough. For one thing, as mentioned, the innate immune pathways have been around a lot longer, and pathogens have had more time to undergo selection for strains that have stumbled into ways to evade or interfere with these mechanisms. The adaptive immune system (AIS) is a more recent development, if by “recent” you can accept 500 million years or so, about the time that fishes with jaws started showing up in the fossil record. There have been some refinements since then, but it’s basically sharks on up. The AIS builds on the innate immune system and adds some extraordinary power and specificity.
The big puzzles in immunity were figuring out how the body could recognize such a wide variety of pathogens, how it was able to “scale up” a response (which can take a few days), and how the memory of such infections is maintained afterwards. The key was realizing that all of us are carrying, at all times, a gigantic combinatorial library of specialized Y-shaped glycoproteins (estimates vary, but probably around ten billion different ones out of a possible suite of a trillion or so) whose function is to hang around until something shows up that one or more of them can bind to. These are the antibodies. They’re carried around on the surface of B cells, and every human being has a somewhat different collection of them. The mechanisms by which our rather limited genome is able to produce such a huge variety of proteins are beyond the scope of a single blog post, but they’re pretty damned impressive, and medicinal chemists who have done combichem and/or DNA-encoded libraries will find a shock of recognition awaiting them when they read about what goes on.
A substance (generally a protein or something bound to a protein) that binds to one of those antibodies is called an antigen, and it’s estimated that only a few B cells respond at first to a new one. But that sets off another really impressive part of the process, clonal selection. When an antigen binds, that surface antibody/antigen complex gets taken back into the B cell, and the antigen itself is chopped up and sent back up to the surface of the B cell as part of its MHC presentation. Then when a helper T-cell binds to that primed B cell, the B cell gets stimulated to divide rapidly. Some of these become plasma cells, which produce large numbers of copies of the antibody that set off the whole process (and release these into the bloodstream and the lymphatic system), and some of which hang around as memory B cells.
And there you have some answers to the earlier immune puzzles: the body is able to recognize so many antigens because we constantly carry around a ridiculously huge variety of antibodies on our B-cells, far more than will ever be activated in any individual’s lifetime. And we “ramp up” the response to such antigens by selecting out the ones that hit and having their carrier cells multiply and produce huge numbers of those particular antibodies. And finally, some of those cells are specifically designated to stay behind, surviving for decades, as a repository of Stuff That Worked That One Time, just in case that particular pathogen should show up again.
I’m leaving out (for now anyway) ungodly amounts of immunology, such as the various functions those antibodies have and the many cell types that carry out their duties in response to them. That’s because I want to go to the things that are making headlines now, the antibody-based blood tests that many companies are working on and presenting to the FDA for approval. The quick point-of-care ones are generally lateral flow assays, whose appearance will be familiar to anyone who’s taken or seen a pregnancy test. You can set these up several different ways, but they all depend on antibody recognition and some sort of colorful indicator.
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!)
So that’s the antibody test situation. There are several companies that have developed point-of-care tests as described above and have FDA allowance (not full approval yet) for use subject to those qualifications mentioned above. Those are definitely useful here in the short term, and as we learn more about the serology and the long-term immune response could be very useful indeed. We need time, and data. As always.