There are basically two types of hypersensitive reactions. Immediately hypersensitivity and delayed hypersensitivity.
Immediate hypersensitivity reaction occurs after ingestion or Inhalation of an allergen resulting in a immediate reaction such as difficulty in breathing, hives , swelling of the mucous membranes. These are signs of a true MEDICAL EMERGENCY and that person must seek immediate medical attention!
Delayed hypersensitivity reactions usually occurs when an allergen comes in contact with the skin and is absorbed as in the case of poison ivy, poison oak, soaps ,cosmetics and a variety of other chemicals. This allergen induces a contact hypersensitivity reaction. The allergen once absorbed through the skin, a rash appears within 5 to 7 days after contact and 1 to 2 days if re-exposed.
The allergen in poison ivy or poison oak is urushiol and can be washed off your skin, gloves, garden tools and pets.This simple step can prevent a rash from occurring.