Q&A: How does the immune system work?

Note: This interview was originally published in Doing Well, a health news outlet from ASU Media Enterprise and ASU Learning Enterprise. Subscribe to Doing Well to get interviews with health experts delivered to your inbox weekly.

By Mel Moore

Have you noticed lots of people out sick at work or school lately? Maybe you’ve heard sniffling on the bus or felt under the weather yourself. Flu cases tend to spike in November and December, on top of other bugs that may be going around.

The cool thing about the human body is that when it does encounter a virus, bacteria or other disease-causing substance (also called a pathogen), it has systems in place to fight it.

To understand this process, we spoke to Douglas Lake, a professor at Arizona State University's School of Life Sciences and an expert in immunology. He walked us through the different pieces of the immune system and how we can build “immunological memory.”

Note: This conversation has been edited for length and clarity.

Question: What is the immune system made up of?

Answer: The immune system is kind of like a three-ring circus. There are various types of white blood cells. The first (are) phagocytes that engulf and digest microbial pathogens. Another, called a B-cell, secretes antibodies (that) coat bacterial pathogens or neutralize viruses. They actually prevent the virus from getting inside your cell. You have hundreds of millions of different kinds of B-cells that are all devoted to secreting different antibodies.

Then there are T-cells, (which) are kind of like the bosses of the immune system. There are multiple different types; they’re the brains of the outfit. T-cells recognize when other cells are infected. One type of T-cell, a cytotoxic T-cell, would recognize when a regular cell (is) infected. It would bind to that infected cell, then kill that cell, and leave a cell that’s not infected alone. There’s also helper T-cells; sometimes they help the B-cells.

Inflammatory (T-cells) increase inflammation. Those cells are important because sometimes we need a really acute inflammatory process to subdue an infection, but other times they can become problematic. If you have too much inflammation, then you might have to take some medicine.

There’s another type of T-cell called a regulatory T-cell, (which) are kind of like the policemen of the immune system. They say, "If you’re a cytotoxic T-cell, you want to kill things, and we’re going to tell you when to stop killing." They secrete cytokines that tell other cells, "Hey, this isn’t a spot where you really need to be inflammatory, so go back to where you belong, in a lymph node." We have lymph nodes all over our body, that’s where white cells reside and communicate with each other. In a nutshell, that’s the immune system.

Q: Are there specific factors that affect our immune response and our ability to fight back against bacteria or viruses?

A: If you have a healthy diet in general, your immune system — if you don’t have some sort of underlying genetic immunodeficiency — does pretty well. Some people walk around with immune deficiencies that they really never know about. Maybe they’ll get a few more colds per year than someone else, but someone can have an immune deficiency and live a perfectly normal life.

Q: It seems like all of those typical recommendations that providers give — a healthy diet, avoiding alcohol, not smoking — are the ways that we can strengthen our immune system.

A: Right. There’s probably not something — even though you’ll see it on lots of products in the drugstore or the grocery store — that can strengthen the immune system. Unless you have an immune deficiency, and in that case, we’re not really strengthening it; we’re just bringing it back up to speed.

Q: What types of things trigger an immune response?

A: When you’re sitting on an airplane, and the person is coughing like crazy next to you, you might end up breathing in some viruses ... (or) maybe it’s a bacteria that you come in contact with. We have receptors on the surfaces of our white cells that recognize bacterial (or viral) structures, and they say, "This isn’t like anything I see in my host. This is a structure that’s different, so I’m going to respond to it. I’m going to make inflammatory signals here." The inflammatory signals might cause other white cells to come in and say, "There’s a little infection going on here. All these cells that are lined up along your airway have been infected, so I’m gonna kill all those ones that are infected, but I’m going to leave all the ones that aren’t infected alone."

In the process of generating an inflammatory response to the virus or the bacteria, those phagocytic cells are going to secrete messengers — we call them chemokines — that call up your B-cells and T-cells to come and very specifically attack whatever virus or bacteria it is that you’re infected with.

Q: What are the most common ways viruses and bacteria enter our bodies?

A: We all touch things, so the best thing you can do during cold and flu season is not to touch your face or put your hands on your lips or your eyes, because bacteria and viruses are everywhere. You can get them through your airways, (or) you can eat contaminated food.

Q: Are there other organ systems, apart from the immune system, that help fight off harmful bacteria?

A: Your microbiome is pretty important in fighting off pathogens. One example is that patients might take a couple courses of antibiotics, and then suddenly they’ve got a yeast infection. Antibiotics are systemic — you take them, and they kill off your normal microbiome as well. Just like in Phoenix, it’s all about real estate. When you kill off your normal microbiome, pathogens say, "Oh, well, here’s some space, so I might as well proliferate (or multiply rapidly)."

Q: Does the body remember how to fight off pathogens it has already encountered?

A: That’s the really cool thing about the immune system ... it remembers. It’s called immunologic memory. And that’s why vaccinations are so important — we can take just the part of the virus that’s involved in infecting the cell, and you can be immunized against that. You’ll make antibodies that bind to that spot on the virus. If you do get infected with that particular virus — and I’m using COVID as an example, because that’s our most recent pandemic — they’ll make antibodies that will bind to that viral spike that will physically prevent the virus from getting into your cells.

You make antibodies, but you also make T-cells, and the T-cells also remember the infection. So you respond to an infection, and after your white cells are finished eliminating that infection, there’s always about, let’s say, between 1–10% of those cells that will say, "Well, I’m just gonna sit here in the lymph node for a while, and I’m gonna remember this. You may encounter this again in a month. And if you do, we’re going to be ready and respond much quicker." So immunological memory is one of the coolest things about the immune response — that’s why we can vaccinate people against measles or bacteria that causes pneumonia.

Q: What are the different ways vaccines can build immunological memory?

A: There are different types of vaccines. For example, most people get a tetanus shot at some point in time. Your tetanus shot is against a toxin that the bacteria produces. You’re not immunized with a toxin, we detoxify it — you’re immunized against that protein. So you make antibodies that neutralize the toxic part of that protein.

We have other vaccines, like measles, mumps and rubella. The MMR vaccines are live vaccines, but they’re attenuated, (or weakened). The polio vaccine is like that as well. Those vaccines are actually wonderful, because what they do is they don’t make people sick, but they kind of go through the same pathway as a regular infection. Your immune system says, "Well, this is something foreign; I’m going to respond against it." But because the vaccine is a weakened form of the virus or the bacteria, it doesn’t make you sick. Maybe you get a sore arm or you don’t feel good for 24 hours, but that is way better than getting sick. Then your immune response is primed — it has that immunologic memory.

There’s these new types of vaccines that we just had with COVID called mRNA vaccines. RNA vaccine research had been going on for 10, maybe 15, years prior to COVID. RNA makes proteins in a cell. If we immunize people with RNA, we don’t have to make the protein outside of the cell — we can use the body, and then you make an immune response against the protein. If that protein is (connected to) a virus, then you’re immune to the virus. If a virus mutates quickly, it’s easier to keep up if you have an RNA vaccine.