My brother and I both had the luck to fall into some poison ivy this weekend, and while I screamed in pain and was full of rashes, he barely had a scratch.
That is very unlikely. Poison Ivy contains urushiol, an oil which can cause rash and itching. Some people are less sensitive than others and their T cells have a milder reaction.
Usually this reaction doesn’t occur on the first exposure. It’s an example of a hapten, which is a molecule that generally doesn’t initiate a full blown immune response on initial contact, as an allergen would. Instead it binds to proteins in your body before becoming an allergen, which on subsequent exposure will then initiate a delayed Type IV hypersensitivity reaction.
The reaction to poison ivy is an immune response that requires prior exposure and the development of specific immune T cells. It is true that sensitivity to urushiol varies across populatio and the reason for this variance is thought to be both genetic and environmental.
According to this article (https://www.niehs.nih.gov/research/supported/sep/2017/protein-poison-ivy-itch/index.cfm) the specific protein involved with strong response is IL-33.
„An NIEHS grantee and colleagues found that the immune system protein interleukin 33 (IL-33) plays a key role in triggering itch from poison ivy, the most common allergic reaction in the U.S. The new findings could lead to treatments for people allergic to urushiol, the oily sap found in poison ivy, poison sumac, and poison oak.”