During a recent hike in the woods I rubbed up against some poison ivy bushes. Itching and blotches appeared on my skin about 4 to 8 days after the exposure. I’ve found this delayed reaction to be typical for me and poison ivy exposure, and I think it’s not uncommon for others as well.
I’ve always wondered why it takes so long for symptoms to appear. I react to other allergens almost instantaneously. Anybody know why poison ivy is different?
Thanks all, in advance.
Poison ivy (Rhus) is a urushiol induced reaction. The compound binds to some skin cell proteins causing the body to react to it as a foreign body with a T-cell response. (Compunds that do that are called “haptens.”) T-cell responses take a few days to mount; cell clones have to get selected and grow.
Many other contact dermatitis reactions are caused by the physical irritation itself or are antibody mediated. (Extant IgE mediated causing rapid onset of a hive type rash that often goes away quickly as well.)
Delayed hypersensitivities result when the cells which precipitate the reaction need time to gear up the production of the mediators for the reaction.
Immediate hypersensitivities are “immediate” because the mediators already exist in enough quantity to trigger an immediate reaction.
I am using the term “mediators” to include everything in the response chain, including various cell types, immunoglobulins produced by cells (themselves in turn often having their production triggered by the offending antigen) and chemical mediators.
Note that it’s possible for a first exposure to an antigen to elicit a delayed or trivial response, and once all the mediators are in place, a second exposure might elicit a more immediate or significant response.
This is a vast oversimplification, but if you look up “delayed” and “immediate” hypersensitivity, you’ll get a good idea what I’m talking about.
ETA: Sorry DSeid; did not notice you already covered it.
I suspect that I had a lot of contact before my first outbreak, then, because for the several years that I kept encountering it, I would develop a rash within hours, (it seemed like minutes), from the contact. I had never heard of a “delayed” reaction to urushiol before this thread.
Can’t help, as I am immune to all poison ivy type plants.
Sorry.
