As in anything, there are multiple causes. DH was telling me that peanut allergy is rare in some country (India?) and they feed the babies peanut containing foods from infancy. Cause or correlation?

I think there was a study that babies can be sensitized to peanuts in utero so if there is a family history, you should abstain from peanuts/nuts during pregnancy.

the immune system randomly rearranges gene segments to produce your repertoire of antigens you can respond to. My theory is that if you are exposed to an antigen when the immune system is naive, those immune cells are a larger proportion of the repertoire and therefore might increase allergy to that antigen. But if the food is given later, when the immune system has setup adequate tolerance measures in the gut, when those same antigenic foods are eaten, the immune system will not react to it as readily b/c those sells will be a significantly smaller proportion of the repertoire due to all the other antigens the child has been exposed to. Every time an antigen is encountered, the immune system expands the cells reactive to that antigen. Hence, allergic reactions get worse upon subsequent exposure b/c more cells are responding and responding more vehemently. the only explanation for kids who went into shock after eating nuts for the first time was that they were sensitized in utero. It has seen been shown that babies in utero can mount an immune response to vaccines given to the mother if given during pregnancy.