Question: Why do people get allergic reactions and rashes to certain things? Why doesn't everyone (or everyone in a family) get the same reactions?

Susan Cartwright answered on 21 Jun 2015:

Allergic reactions are basically your immune system being too enthusiastic for your own good – it is viciously attacking a foreign substance that isn’t actually harmful. Sometimes, as in some people with acute nut allergies, the allergic reaction is actually life-threatening, inducing a condition called anaphylactic shock which causes so much swelling and inflammation that patients die because they cannot breathe or because their blood circulation fails.

There is definitely some genetic component to this: for example, one study of peanut allergy in twins, by Mt Sinai School of Medicine in New York, found that, in cases where one twin had peanut allergy, the other twin had it in 9 out of 14 pairs of identical twins, but only 3 out of 44 pairs of non-identical twins. Identical twins have exactly the same DNA, whereas non-identical twins are no more similar in their DNA than any other pair of brothers or sisters. The fact that the identical twins were much more similar in their allergic response points to a strong genetic component – but the fact that the similarity was not 100% (in 5 of the 14 pairs, the other twin *didn’t* have peanut allergy) shows that it is not entirely genetic.

The genetics of the immune system is ferociously complicated and variable. A study done by King’s College London in March 2015 found “over a hundred gene variants associated with different types of white [blood] cells all with different immune functions.” This means that even closely related people will have different immune systems, since you inherited half your genes from your mum and the other half from your dad. This is one reason why even people in your immediate family don’t share your allergies (my mother is dangerously allergic to penicillin, as was at least one of her brothers, but I’m not).

Another complicating factor is that your immune system is strongly affected by your environment: this is why not all the identical twins had the same response to peanuts. In fact, another twin study by Stanford University showed that most immune responses are more strongly influenced by environment than heredity – nearly 60% of the immune responses they studied were almost entirely independent of genetic factors. They also found that the older a pair of identical twins were, the less similar their immune responses were – this is because in early life, identical twins have very similar environments (they live in the same house, are fed the same foods, usually share a room), but as they become adult their environments diverge (they leave home and live separate lives).

The incidence of allergic reactions, and allergy-related conditions like asthma, seems to be increasing (when I was a child, allergy to nuts was pretty much unheard of, whereas now it seems that nearly every class has at least one kid with a nut allergy). Many medical researchers think that this is because our homes are much cleaner now than they used to be, and children spend more time indoors. This means that your immune system does not get exposed to nearly as many minor infections and invasions of foreign substances in early childhood as it used to (and evolved for). Consequently, when it does get challenged by foreign substances, it over-reacts, producing an allergy. Related to this, the Stanford university study I mentioned found that one of the most important factors affecting the immune systems of the people they studied was whether or not they were infected by cytomegalovirus, a usually harmless virus that more than half of us carry all our lives without knowing it or being harmed by it (though it can be dangerous to people with damaged immune systems, such as those with HIV and those who are taking immune-suppressing drugs after a transplant). The immune systems of identical twins were much more different from each other in cases where only one twin was infected than they were in cases where both or neither were.

So, to summarise: you get allergic reactions because your immune system over-reacts to harmless substances. Different people, even within a family, don’t all react the same because the genetics of their immune systems are different (except if they are one of a pair of identical twins) and also, perhaps even more, because their immune systems have not encountered the same set of challenges from the environment over the course of their lives (even if they are one of a pair of identical twins).