We frequently get asked questions about allergy to cats so.... I have interrogated a friend who just happens to be a Registrar in Immunology at Southmead Hospital - here is what he had to say......
Cats secrete a specific protein in their saliva which produces an allergic response in certain people. With frequent grooming the saliva,and therefore the protein, gets spread all over the cat's coat and all over the house as the cat moults. Longhaired cats tend to be worse than shorthairs.
Cat owners carry the protein on their clothes too so it's very difficult to avoid wherever you go.
Most people allergic to cats have an immediate response - rhinitis (runny nose) and streaming eyes, although some people can have a less severe reaction that occurs several days later.
Also, people can develop an allergy after a lifetime with cats - if you have a cat-break for several months eg going off to University, you may well find you are allergic when you return home to puss. Solution? Sounds stupid but take the cat's blanket with you!
You can be de-sensitised to cats but it is a long process - weekly injections of low dose cat allergen for 6 months, then monthly injections for 3-5 years. It usually works but there is no guarantee. Not to be undertaken lightly.
Also, it is not available on the NHS unless you work with cats - eg veterinary nurse. Pet owners do not qualify!
CHILDREN - Any allergy which causes rhinitis in children (cats, pollen, dust mites whatever) has the potential to cause asthma. Much as it pains me to say it, the medical advice is: if your child is allergic to cats please re-home your cats.
GOOD NEWS - the Americans have bred a cat which does not secrete this protein. BAD NEWS - it will cost you £2000 per cat and there is a year's waiting list!