The Science Behind Allergic Reactions & Viral Infections
If you’re reading this, there’s a good chance you already have a general understanding of the science behind allergic reactions. Today we’re going to go into (a little) more depth about exactly what’s going on. There are also parallels with viruses, making this topic particularly relevant today.
The Immune System: An Overview
Allergies are the result of your immune system treating normal substances such as pollen, peanut, and latex-like pathogens: microscopic disease-causing creatures such as viruses or bacteria. So, to understand allergies, we have to understand how your body defends against pathogens.
The body’s first line of defense is the barriers and mechanisms that keep pathogens from getting in: skin, coughs, tears, sneezes, and digestive enzymes in your stomach, to name a few.
If these barriers fail, the body then relies on white blood cells: the soldiers of the immune system. The most relevant white blood cells for allergies are the T cells and B cells, which allow the immune system to target specific pathogens. Pathogens have little pieces on them called antigens, which B and T cells use to identify harmful intruders. T and B cells can recognize these antigens with special structures along their perimeters.
Pretty cool, right?
The special structures on B cells are Y-shaped proteins called antibodies (or immunoglobulin). The tip of each branch in the Y can bind to the antigen portion of the pathogen, matching it like a lock to a key. This metaphor might sound familiar to you—our technology works on the same principle, although we use polymers instead of antibodies. The body produces billions of these B cells, and they all have different antibodies, each made to fit a different antigen. This incredible diversity is how your body can respond to different types of pathogens like tetanus, smallpox, and of course, COVID-19. You can imagine it as a very large game of Tetris, and your body only needs one piece to fit in order to start producing antibodies to fight the virus.
Creating B and T cells is an incredibly complex process, which leaves a lot of room for error. One way things can go wrong is if these cells start targeting things they shouldn’t. For example, autoimmune disorders like lupus or type 1 diabetes are the result of antibodies targeting healthy, normal cells. Likewise, a person with allergies has antibodies that target the allergen, turning the full power of the immune system against what should be a harmless substance.
Once the T and B cells recognize and bind to the antigen part of an allergen, the T cell activates the B cell, which does a couple of things. First, the T cell tells the B cell to copy itself. Some of those copies turn into antibody factories, releasing thousands of identical free-floating antibodies per second that attach to the allergen. The other copies become memory cells, which live on in your body, allowing you to react more quickly if exposed again to the allergen.
Side note: these memory cells are how you develop immunity to reinfection with the same disease. This is how most vaccines work—you get injected with a pathogen that has been modified to be harmless, which lets your body create memory cells for that pathogen without posing danger. Then, when your immune system encounters the real thing, it can react quickly enough to subdue the pathogen before it can do any harm.
The Allergic Response
Phew! Still with us? Good.
You now might be wondering, OK, but what’s so bad about antibodies attaching to allergens and reproducing themselves?
Antibodies identifying and binding to allergens are only part of the body’s immune response. When the T cells activate the B cells, they also force the B cells (the ones mass-producing antibodies) to change the stem part of those Y-shaped antibodies. In humans, the stem will change to one of five types, each of which is specialized to deal with different types of antigens. For most allergies, the stem changes to immunoglobulin E, or IgE for short.
Antibodies with this IgE stem can then attach to certain types of white blood cells, most importantly mast cells, while simultaneously attaching the tip to an allergen. The mast cell then releases a cocktail of molecules and proteins that act as signals calling for backup support from the rest of the immune system. You may have heard of one of these signaling molecules before: histamine. It’s why antihistamines are commonly used to treat allergies.
The most basic way for the signals to “call for help” in the immune system is to increase blood flow to the “infection” site, which they do by widening the blood vessels and making holes in the blood vessels for white blood cells to travel through. Some of the other signaling molecules are chemicals that attract the white blood cells out of the blood and into the target area. The body will also increase mucus production.
The symptoms we all know from allergic reactions result from these processes occurring in various parts of the body. Increased blood flow in the area leads to swelling, redness, and heat. Inflammation in the stomach can cause abdominal pain and stomach cramps. Increased mucus production can lead to a runny nose. Fluid leaking from the blood vessels under the skin causes hives to form. And so on!
Anaphylaxis: The Worst-case Scenario
The response becomes life-threatening when the immune response threatens the body’s ability to breathe and circulate blood. The increased blood flow to the “infected” area can cause low blood pressure, which can result in dizziness and muscle contraction around the lungs, which can make breathing more difficult.
Anaphylaxis is the most severe form of an allergic reaction. The symptoms of anaphylaxis are essentially the symptoms above in overdrive.
Anaphylaxis becomes fatal in two ways: either the constriction of the lungs prevents enough oxygen from reaching the body (asphyxia), or the widening of the blood vessels prevents adequate blood flow to tissue (anaphylactic shock).
So that’s the (sort of 🙃) simplified version of what we know about allergic reactions. However, there’s still a lot more we have to learn. For example, why do only some foods trigger allergic reactions? Why are some more severe than others? Why do some people have allergies and not others? Why is the incidence of food allergy on the rise?
On a microscopic level, there’s still a ways to go in understanding the mechanisms of the body’s allergic response. Hopefully, as the science progresses, we’ll be able to answer these questions more fully and make the lives of allergy sufferers easier. And of course, you’ll have the Allergy Amulet to help you out, too!
- Nazir and the Allergy Amulet Science Team
This piece was reviewed by Allergy Amulet advisors Dr. John Lee and Dr. Jordan Scott.
Dr. John Lee is an allergist and the Clinical Director of the Food Allergy Program at Boston Children’s Hospital.
Dr. Jordan Scott is an allergist and operates several private allergy clinics throughout the Boston area.