People are allergic to many things - including cats - and these allergies can manifest themselves in different ways. Some people will be mildly affected, feeling itchy, snuffly or sneezing; others have severe skin reactions, and a few may experience a serious asthma attack.
These reactions occur because the person's immune system reacts to particular proteins (antigens) derived from an animals or plants to produce antibodies and/or other chemicals to protect the body. One of the normal actions of antibodies is to attack foreign organisms such as viruses, bacteria and parasites and to stimulate the body to release histamine to defend itself. In allergic people, histamine is inappropriately released after exposure to antigen that is not a threat (allergen). This can irritate the lungs, nose, skin and other tissues - hence the common reactions of sneezing, coughing, wheezing or itching.
The main trigger for the allergic reaction to cats is a protein which is secreted in saliva and in the skin of all cats (not in the fur or dander). It can be found on the coat because the cat grooms itself using saliva. It is also shed in urine or faeces.
The signs of reaction can occur if the allergen is inhaled or a person strokes a cat, cleans its litter tray or even sits where the cat has been sitting. All breeds of cat produce allergens, but some may produce more than others - it can be a case of trial and error to ascertain which cats you react to. Although there is no scientific reason for it, many people seem to react more to longhaired cats - perhaps because more allergen builds up on their fur or accumulates because of more hair around the house. Likewise, even cats with little hair such as Rexes or Sphynx cats may still cause a reaction – there is no easy answer to allergy-free cat keeping.
If you have a cat and are mildly allergic to it, then keep it out of your bedroom and off the bed and keep the house (carpets, curtains and cushions) well cleaned - a build-up of allergens is what can trigger reactions. Wooden or tiled floors with washable rugs are also much more cleanable than wall-to-wall carpets. Vacuuming is thought to be of limited help as it can stir up allergens and good ventilation is essential. Experts may suggest washing the cat regularly - they are probably not cat owners or they would realise the difficulty of doing this! There are products on sale which claim to reduce allergens if they are applied to the cat's coat regularly - at present there seems to be little information on how well these work.
There are drugs available from the doctor and over the counter, which can help ease the reactions but their efficacy is obviously dependent on that person's particular degree of reaction. Antihistamines are usually the mainstay of treatment for sneezing and itching while an inhaler and even steroids are sometimes required. Unfortunately antihistamines are often sedating (you may not be able to drive) and may interact with other medications so first seek advice from your doctor.