I have some corrections to the posts about poison ivy/poison oak. At the end I will give my credentials. I got my info from many clinical studies, not from internet posts or articles, because the same wrong info is circulating around and around, and not being corrected or updated.
The allergenic oil in mangos is resorcinol. The allergenic oil in poison ivy/oak, including poison sumac, is urushiol. They are together in the botanical family, but not genus.
There is enough of a similarity between the two oils (although urushiol is considered to be stronger) that if you are allergic to one, you are allergic to the other. It follows that if you develop a tolerance to one, you develop a tolerance to the other, although you may not respond to mango exposure but get a reaction from PI/PO if it is a strong exposure.
Native Hawaiians usually have no reaction to mangos, but tourists do. This is from lifelong exposure. The skin is regarded as being the problem more than leaves, because the fruit is eaten a lot. The resin canals (the allergenic oil is in the resin) are in the skin, and when broken the resin leaks out. The oil can also migrate into the pulp that normally would not contain the oil. The highly allergic can get a reaction from even mango juice because of this leakage.
Most folks have a threshold. Pass that with exposure, and you are allergic. More exposure can increase the allergy, but certain amounts of small exposure consistently has been shown to sometimes cause a buildup of tolerance within the immune system.
All this is very complicated, not easily thrown out in a few words.
Two studies on jewelweed have not shown any benefit, but easterners often swear by it.
Yes, Technu works to remove the allergenic oil, but contains a somewhat toxic petrochemical, but it is smart to carry something for immediate oil removal. If you dont have anything, dig down to the real soil under mulch immediately after exposure and scrub the dry clay on exposed skin. Clay pulls oil to itself. Straight water does not cut oil. You need a strong soap, alcohol, etc. Carry bentonite clay with you. It's works better than dirt clay.
That "certain curve"in a bare winter branch is the plants reach upward toward the sun as it grows toward open paths. Good for you in noticing that. People develop a automatic poison ivy/oak sensor over time. Certain growth and botanical patterns are very distinctive.
Cashews have allergenic oils, cardol and anacardiol which are in the shell of the nut. The oil of the nut itself is harmless. Imported cashews are shelled first, but sometimes the oil gets on the nuts. They are all heated a bit, because this makes the oil harmless. All cashews have been heated, even though they may be called raw. The same as with mango oil—if you are allergic to poison ivy/oak, you are probably allergic to cashew oil.
My credentials: I wrote "The Poison Oak & Poison Ivy Survival Guide."