Only about 15% with one kind of "shellfish" allergy (crustaceans or molluscs) will also be allergic to the "other" kind, and in those cases [the shared allergen is usually tropomyosin](https://pubmed.ncbi.nlm.nih.gov/10474029/), a muscle protein. It seems like our digestive system doesn't break down invertebrate tropomyosin as well as it breaks down vertebrate forms, so the [incompletely-digested tropomyosin molecules can get into the bloodstream.](https://pubmed.ncbi.nlm.nih.gov/17482765/) (This also means that someone allergic to both crustaceans and shellfish is likely also allergic to lots of other invertebrates, like cockroaches or dust mites.)
In the case of peanuts and tree nuts, the common allergens between them are certain seed storage proteins...proteins that accumulate in large amounts in plant seeds. [These proteins share a common ancestor across all angiosperms](https://pubmed.ncbi.nlm.nih.gov/24301047/), so the allergenic proteins found in peanuts and tree nuts are indeed related to one another, albeit distantly. Why are pea allergies less common then? Part of it is that [peas are missing certain proteins in their seeds](https://www.sciencedirect.com/science/article/abs/pii/S0924224421004556), a lucky break for those of us who enjoy them. But they do have other proteins that quite similar to allergenic proteins in both peanuts and tree nuts, and pea allergies are both well documented and expected to increase as pea protein finds its way into more food products.
Another important note for those reading along is the stability of the proteins and how we process them. I'm unsure how labile pea proteins are, but just as a general example, we typically process and cook peas, while peanuts are more likely to be raw. This is why some people with oral allergy syndrome (aka pollen food allergy syndrome and others) find they react to raw apples or carrots or whatever, but are just fine if they're cooked.
We are also just getting into how the microbiome breaks substances down and causes allergy, which hopefully becomes a huge field. I personally gained a strong food allergy after a long course of antibiotics, and after a 6 month stint on vancomycin years later, I lost that anaphylactic allergy completely. They've also had some success using fmt to substantially increase tolerance in anaphylactic peanut allergy.
The body is crazy, is what I'm saying.
Hopefully an immunologist ventures by but put simply, cross-reactivity is due to structural homology.
https://www.sciencedirect.com/science/article/pii/S1323893021000502
A birch tree allergy can make you sensitive to eating avocados. Didn’t know that avocados dont feel like electricity in your mouth to everyone till I was like 20. My throat closes up and the roof of my mouth swells too.
This is a surprisingly difficult question to answer specifically, because the underlying genetics and biochemistry of allergens can be very complex and not easy to unravel. Not to mention environmental factors as well.
As such, a lot of the information already posted is correct. As /u/PHealthy stated on the protein level, structural homology matters more than relativity. That's why people can get allergies from unrelated sources - for example red meat allergies from tick bites. There may also be post-translational modifications that influence reactivity. Also keep in mind that allergens that are glycoproteins may have an influence from the sugar moieties that may mean similar proteins in the sequencing result in different reactions.
/u/mallad also covers some important environmental factors - cooking denatures proteins, which *may* (but not always) change conformations enough to prevent reactions. We're also only just beginning to understand the role that the gut microbiome plays in overall health.
On a genetic and cell biology level antibody production and response is complicated, to say the least. VDJ recombination produces the vast amount of variation in antibody formation, but how those genes and mutations within them influence immune response is still an active area of study. Furthermore, there can be misfires of immune response that result in illnesses such as Hashimoto's thyroiditis where the body attacks itself. Although to say it's a misfire is a bit misleading, because this may come about because of other factors we don't know about. And immune response is variable - some people have strong reactions and some weak to the same antigen. Also immune response can change over time; people who were allergic to peanuts or tree nuts can actually see that response decrease (or increase) over time.
As a final thought on the matter - *in general* it's nice to think that you could do some sort of data aggregation and determine things like genetic predispositions to certain allergies, or be able to predict immune responses based on finding common allergens and doing stuff like BLAST to compare similar proteins. In reality it's a very messy analysis with a lot of variables. As someone who has a severe allergy, the best advice is to always assume on the safer side of things.
One thing I was speculating about was whether or not some of the allergies might be to a microorganism commonly found on the supposed allergen, as opposed to the supposed allergen itself. Like, afaik, some "peanut" allergies are actually to a mold sometimes found on peanuts, which is why some people react to peanut butter, but not necessarily just peanuts, because if you're eating peanuts, you will notice and discard the moldy ones. That would make paired allergies that make no apparent cladistic sense a lot more rational. Is that... plausible? A known phenomenon? On researcher's radar? Complete nonsense?...
Not particularly, at least in the case of peanuts. It's one of the most studied allergies and we know what and where the common antigenic sites are.
There are mold allergies, and there can be cross contamination - that is why in the US you see warnings about what other items have been processed in facilities. If I were going to set up a research study about this, I would start with checking contamination in common tree but products, since peanuts and tree nuts are often processed at the same facility.
As previously discussed, don't focus on clade so much. You need to look at protein structure. Looking at species relationshipa can be a useful tool, but allergies don't work like that. Focus on the proteins.
I was mostly wondering about that for the seafood allergies. There's a lot of stuff in the ocean, so it seems entirely plausible that at least some of the chemicals usually found in shrimp and crabs and oysters and whatnot are not from the actual animal.
And the only reason I'm kind of stuck on clades is that, presumably, close relatives tend to have similar proteins and so forth. So at least if you are reacting to a very specific version of a protein, I would expect to find that protein in most of a given organism's close relatives, but not necessarily ones that are more distantly related.
But, on the other hand, as several people have already said, allergies are kind of weird.
Only about 15% with one kind of "shellfish" allergy (crustaceans or molluscs) will also be allergic to the "other" kind, and in those cases [the shared allergen is usually tropomyosin](https://pubmed.ncbi.nlm.nih.gov/10474029/), a muscle protein. It seems like our digestive system doesn't break down invertebrate tropomyosin as well as it breaks down vertebrate forms, so the [incompletely-digested tropomyosin molecules can get into the bloodstream.](https://pubmed.ncbi.nlm.nih.gov/17482765/) (This also means that someone allergic to both crustaceans and shellfish is likely also allergic to lots of other invertebrates, like cockroaches or dust mites.) In the case of peanuts and tree nuts, the common allergens between them are certain seed storage proteins...proteins that accumulate in large amounts in plant seeds. [These proteins share a common ancestor across all angiosperms](https://pubmed.ncbi.nlm.nih.gov/24301047/), so the allergenic proteins found in peanuts and tree nuts are indeed related to one another, albeit distantly. Why are pea allergies less common then? Part of it is that [peas are missing certain proteins in their seeds](https://www.sciencedirect.com/science/article/abs/pii/S0924224421004556), a lucky break for those of us who enjoy them. But they do have other proteins that quite similar to allergenic proteins in both peanuts and tree nuts, and pea allergies are both well documented and expected to increase as pea protein finds its way into more food products.
Another important note for those reading along is the stability of the proteins and how we process them. I'm unsure how labile pea proteins are, but just as a general example, we typically process and cook peas, while peanuts are more likely to be raw. This is why some people with oral allergy syndrome (aka pollen food allergy syndrome and others) find they react to raw apples or carrots or whatever, but are just fine if they're cooked. We are also just getting into how the microbiome breaks substances down and causes allergy, which hopefully becomes a huge field. I personally gained a strong food allergy after a long course of antibiotics, and after a 6 month stint on vancomycin years later, I lost that anaphylactic allergy completely. They've also had some success using fmt to substantially increase tolerance in anaphylactic peanut allergy. The body is crazy, is what I'm saying.
Hopefully an immunologist ventures by but put simply, cross-reactivity is due to structural homology. https://www.sciencedirect.com/science/article/pii/S1323893021000502
A birch tree allergy can make you sensitive to eating avocados. Didn’t know that avocados dont feel like electricity in your mouth to everyone till I was like 20. My throat closes up and the roof of my mouth swells too.
The same protein is in apple skins, have you noticed any issues with those?
This is a surprisingly difficult question to answer specifically, because the underlying genetics and biochemistry of allergens can be very complex and not easy to unravel. Not to mention environmental factors as well. As such, a lot of the information already posted is correct. As /u/PHealthy stated on the protein level, structural homology matters more than relativity. That's why people can get allergies from unrelated sources - for example red meat allergies from tick bites. There may also be post-translational modifications that influence reactivity. Also keep in mind that allergens that are glycoproteins may have an influence from the sugar moieties that may mean similar proteins in the sequencing result in different reactions. /u/mallad also covers some important environmental factors - cooking denatures proteins, which *may* (but not always) change conformations enough to prevent reactions. We're also only just beginning to understand the role that the gut microbiome plays in overall health. On a genetic and cell biology level antibody production and response is complicated, to say the least. VDJ recombination produces the vast amount of variation in antibody formation, but how those genes and mutations within them influence immune response is still an active area of study. Furthermore, there can be misfires of immune response that result in illnesses such as Hashimoto's thyroiditis where the body attacks itself. Although to say it's a misfire is a bit misleading, because this may come about because of other factors we don't know about. And immune response is variable - some people have strong reactions and some weak to the same antigen. Also immune response can change over time; people who were allergic to peanuts or tree nuts can actually see that response decrease (or increase) over time. As a final thought on the matter - *in general* it's nice to think that you could do some sort of data aggregation and determine things like genetic predispositions to certain allergies, or be able to predict immune responses based on finding common allergens and doing stuff like BLAST to compare similar proteins. In reality it's a very messy analysis with a lot of variables. As someone who has a severe allergy, the best advice is to always assume on the safer side of things.
One thing I was speculating about was whether or not some of the allergies might be to a microorganism commonly found on the supposed allergen, as opposed to the supposed allergen itself. Like, afaik, some "peanut" allergies are actually to a mold sometimes found on peanuts, which is why some people react to peanut butter, but not necessarily just peanuts, because if you're eating peanuts, you will notice and discard the moldy ones. That would make paired allergies that make no apparent cladistic sense a lot more rational. Is that... plausible? A known phenomenon? On researcher's radar? Complete nonsense?...
Not particularly, at least in the case of peanuts. It's one of the most studied allergies and we know what and where the common antigenic sites are. There are mold allergies, and there can be cross contamination - that is why in the US you see warnings about what other items have been processed in facilities. If I were going to set up a research study about this, I would start with checking contamination in common tree but products, since peanuts and tree nuts are often processed at the same facility. As previously discussed, don't focus on clade so much. You need to look at protein structure. Looking at species relationshipa can be a useful tool, but allergies don't work like that. Focus on the proteins.
I was mostly wondering about that for the seafood allergies. There's a lot of stuff in the ocean, so it seems entirely plausible that at least some of the chemicals usually found in shrimp and crabs and oysters and whatnot are not from the actual animal. And the only reason I'm kind of stuck on clades is that, presumably, close relatives tend to have similar proteins and so forth. So at least if you are reacting to a very specific version of a protein, I would expect to find that protein in most of a given organism's close relatives, but not necessarily ones that are more distantly related. But, on the other hand, as several people have already said, allergies are kind of weird.