When you talk Poison-ivy, you're talking Urushiol poisoning. Urushiol is very sophisticated oil-based, hydrophobic toxin. When it comes in contact with skin, you have about 15… minutes to wash it off, with a strong surfactant/soap. After that, it bonds at the cellular level (and after that point is no longer "contagious"). It seems to modify skin cells and others, so that your immune system cannot identify them as part of your body, and a T-cell mediated immune reaction commences. In other words -- this is a toxin that induces an allergic reaction. And after 15 minutes, all we can do is treat the effects and wait for it to go away. There are a few vaccines out there, but they aren't generally available, and only work if applied before the contact. .
Note that, once the reaction appears, it doesn't really "spread", even though it appears to. What's happening is that areas that got the biggest dose of urushiol manifest first. Lighter doses show up later. While you can re-infect from cloths and such that picked this up, you can't infect from the lesions. .
People generally get urushiol poisoning with by direct contact with the plant, or from clothing that contacted the plant. Urushiol is super-persistant, so you have to wash the heck out of everything that might have come in contact with it. Dry cleaning works too, in that anything that breaks down oils will do the job. .
About 20%-30% (roughly) of all people don't react to urushiol on first contact, but most seem to develop a sensitivity over multiple exposures. Native Americans seem to be highly resistant to this toxin, likely from evolutionary pressure, as this appears mostly on the North American continent. You pretty much can't go through your life without meeting up with urushiol at least once. In fact, this accounts for about 10% of the US Forest Services medical leave. .
The symptoms are typically a rash with blisters that weep a transparant liquid -- clear or yellowish. Note that this liquid doesn't carry the toxin at all. The rash is extremely itchy. Normally, the rash doesn't appear until 24-48 hours after exposure (and sometimes as much as a few days), and lasts up to 14 days (but more usually 4 or 5 days). Blisters that seep pus instead of a clear liquid are showing indications of a secondary infection -- more on this in a second. .
As this is an immune reaction, it's safe to treat it as one. First, DON'T scratch it -- it'll just make things worse. Epsom salt baths in cool water have been known to be effective -- a little. Also, consider a cortizone ointment on the surface. .
One systemic that works is diphenhydramine -- for this I like a big dose that both puts you to sleep and stops the histaminic reaction. So read the max dosage on the box and go there -- make sure you have nothing else to do that day :} .
I've tried topical painkillers like lidocaine. For me, it does a great job for about 20 minutes and then it's back to normal. It wouldn't hurt to try but I doubt that'll help much. .
If you can't control it with diphenhydramine, baths and cortisone ointment, your doctor can prescribe prednisone -- a powerful anti-inflamatory, and this Does work. Your doctor will tell you this, but be Very careful with the dosage --- you starting with a large loading dose and gradually ease off -- the doctor and the package will explain this in detail. .
Most of the time, your doc will administer prednisone orally. Injectable prednisone works very well indeed too. .
Scratching and general cellular destruction opens the door to a secondary infection -- often staph or strep -- and these are bad news. To avoid this, keep the affected area super-clean (but dont scratch!). If you start seeing pus, treat that area first with antiseptics. If that doesn't work, talk to your doc about antibiotics. .
Now -- here's the rub: .
Prednisone reduces your immune system response (which is why its used against urushiol toxin), which means that it's easier for you to pick up that secondary infection. So you really have to keep on top of this. .
Did I mention no scratching? .
An add on: .
The advise above is excellent but if you aren't really sure what the rash is then I would suggest going to a doctor for a diagnosis. I have gotten poison ivy every year of my life and I always treat it myself except one time I had it so bad that I had to seek medical treatment..
Another Add on:.
Calamine Lotion is also good for something that will prevent itching as well. It is a topical cream. ( Full Answer )