The body reacts to these alien proteins, and the skin becomes inflamed and itchy. Be sure to spray shady areas and tall grass, where mosquitos and fleas hide. I also recommend using one of the new wave of veterinary-sold flea products on your pets. Frontline also works on ticks. Revolution is even better, killing ticks, ear mites, and the mites responsible for that itchiest of skin conditions, demodectic mange.
Revolution also replaces heartworm preventive.
Use antihistamines cautiously with epileptic dogs, and those with glaucoma and heart problems. Don't use with pregnant animals. Vets favor steroids such as prednisone in the form of pills or shots. They're cheap, and usually effective. But they have very bad long-term side effects, including damage to various internal organs, and incontinence.