Allergies can develop to many things, with allergies to plants being very common. As a general rule allergies to plants can be divided into two general categories. For simplicity when we refer to plants in this article, we are including trees, grasses, weeds and shrubs etc.

Contact allergies
These types of allergies involve actually having to come into contact with the offending plant. They can develop at any age, including in the very old and young. Due to the need to come into contact with the plant, these allergies will commonly give a distinct pattern of areas of the body affected. These are the areas of the body with naturally less hair cover and those which tend to contact the ground. The feet and belly are the most commonly affected, although the muzzle (as a dog sniffs the ground) inside of the ear flap (whilst laying down) are also areas which can be affected.
Contact allergens should be understood to be a different thing to contact irritants. A contact irritant is something which could reasonably expect to cause a rash, burn or similar on anyone (i.e. an acid or in the case of plants, poison ivy). A contact allergen maybe perfectly fine for some people or pets to come into contact with (i.e. a grass) but will cause others great problems.