Note: it has been pointed out to me that this article can give the impression that there is only one, all-inclusive shellfish allergy. That is not the case: one can be allergic to just crustaceans or just mollusks. Never assume that someone isn’t allergic to “shellfish” because you see them eating some.
Way back in 1997, my then 18 month old son was enjoying a complicated dish of Asian seafood, when his face blew up like Violet Beauregarde chewing Willy Wonka’s experimental gum. Off we went to the Emergency Room, and after some oxygen and Benadryl, we were instructed to follow up with an allergist. A complicated series of food restrictions followed, we learned that he was allergic to shellfish. This is a very serious allergy which can cause deadly anaphylaxis. Some people are so sensitive that even being in the same room as shellfish is risky. I took the warnings seriously.
Having taken a lot of biology in college, I asked the allergist what exactly “shellfish” meant. After all, it’s a culinary term, not a taxonomic one. Shellfish include species such lobsters and shrimp, but also things like clams and mussels. Why would “having a shell” cause something to hyperallergenic? These were totally different species, and even their shells were made out of completely different materials.
It turns out that no one knew until the mid-1990s. But the pattern was clear, and the allergist was right: people who are known to be allergic to shrimp, for example, are often allergic to clams and mussels, even though there was no clear connection between crustaceans and mollusks, except that they live in the sea. Tuna fish live in the sea too, but they don’t cause reactions in people with shellfish allergies, so the sea isn’t the connection. What could it be?
Allergies are caused by the body overreacting to a protein. Scientists surmised that there must be some protein that crustaceans and mollusks share, and in 1993, they found it: it’s called tropomyosin. In fact, they found several, but tropomyosin is the one that seems to be responsible for most reactions. It’s a protein found in most animals, but it’s found in greater concentrations in mollusks and crustaceans.
It turns out that it’s also found in great concentrations in insects as well, but as few people eat them in our culture, the allergy hasn’t been expanded to “shellfish and bugs” allergy. But it does include things we don’t think of a shellfish, such as octopus and squid. They’re mollusks, just like clams, and they also have the protein in large concentration.
So, in this one instance, the culinary world knew something that science didn’t: the classification of “animals that live in the oceans and have shells” is a useful one, even if it has nothing to do with the evolution of species. As for my son, he’s been able to avoid eating shellfish for 18 years now, and though he’ll likely have to avoid them for the rest of his life, he can at least know why. And also why he should avoid chocolate covered ants and fried crickets that are popping up in novelty shops.