I recently learned about the Alpha Gal carbohydrate that is in every mammal except for humans and apes. Apparently, when a tick bites you, it can inject Alpha-Gal into you and suddenly you become allergic to it. I’ve eaten mammalian meat my entire life, so I know I’ve encountered Alpha-Gal carbohydrates for 35 years. How is it that encountering it in the context of a tick bite can cause an allergy, but eating it for decades does nothing?
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This requires knowing how the adaptive / acquired immune system works. I am not a doctor, but I’m a non-human geneticist, so I’ll do my best. So, we all know that after you’ve had a certain disease and recovered from it, your body is more immune to that disease. Chickenpox is a good example; we usually don’t get flu more than once a year. How does that work?
Well, basically, all the time we have detector cells called T-lymphocytes, T-cells. The job of a ~~detector~~ T-cell (editing to remove some of the wrong bits) is to be on the lookout for its match. Every *T-cell has a certain little thingy on its surface, a receptor, that is looking for a certain protein match.
When the *T-cell finds its match, it *does one of two things, depending on whether it’s a killer T-cell, or a helper T-cell. Killer T-cells go find cells that have their match on the surface, and kill them. Helper T-cells are kind of like messengers: they send signals to attract other cells to mount an immune defense.
(*There’s also a cell type of called dendritic cells that do a lot of helper-type work, they help bring pathogens to T-cells, etc.; and a comment graciously added below talks about that in more detail.)
One of the cells attracted by the T-cells is called the B-lymphocyte, the B-cell. The B-cell also has a receptor on its surface, but when its receptor is matched it has a different role. When a T-cell *activates its matching B-cell, the B-cell multiplies into a whole bunch of what are called memory B-cells. The role of memory B-cells is to simply keep existing and producing antibodies against all the pathogens you’ve experienced before, in case they come again.
What is an antibody? It’s a chemical that flags a pathogen for destruction by other immune system killer cells.
So the way it works is, when a *T-cell matches a pathogen, it attracts a matching B-cell, which becomes a memory B-cell, which produces antibodies to that match, and then the antibodies tag the pathogen to be destroyed by another cell. So far it doesn’t really explain allergies.
Here’s the problem: we have *lots* and *lots* of different types of B-cells and T-cells. Pathogens are constantly evolving, so our bodies have no instincts for what the pathogen is going to look like next. In fact, our genes in our immune cells are constantly recombining to produce *new types* of T and B cells. New matches that the immune system starts to look out for arise all the time.
What do you suppose happens if a match arises for a compound that is normally part of the human body? What if the body starts to attack itself?
Well, turns out, auto immune diseases where the body attacks itself actually *do* happen all the time. You probably know the names of some of these diseases: lupus, psoriasis, celiac disease, even some types of diabetes.
The way this self-attack is normally prevented is by a second instance of the same general system; some T-cells are responsible for detecting things that are normal for the body, and preventing any T-cells and B-cells from attacking them.
So, basically, the body *has two running lists of all the matches, where these lists are basically banks of cells that are primed to treat a certain match a certain way. (For better detail on how the “good list” works differently than the bad list, see the comment by Jkei below.) Matches that are normally supposed to be there are on the good list, and your body prevents itself from becoming immune to those. Things that are known to be dangerous are on the bad list, and the body produces antibodies against them all the time.
So what determines whether something’s on the good list or the bad list?
I do not know of any hard and fast rule, but somehow, alpha-gal ends up on the bad list after a tick bite when it didn’t before. Here are some aspects that are different that may contribute to why:
* Ticks bite directly into the bloodstream. Food passes through the digestive system first, so there could be different dynamics in the amount of alpha-gal one is exposed to via food vs. via a tick bite.
* Ticks don’t *just* produce alpha-gal. Their saliva also contains all sorts of *other* foreign proteins and microbes that are going to co-activate the immune system. These microbes will “attract the attention” of the immune system in a way that normal food consumption simply doesn’t.
* Any tick bite by definition is going to create a small wound, triggering other bodily self-repair responses.
From what I understand, it is context. Your stomach knows what to do with Alpha-Gal but your bloodstream does not. So the blood calls in reinforcements (immune system) to get rid of it. The immune system then adds Alpha-Gal to the “intruder” category, which produces an allergy
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