You may want a cat but you have allergies. You may already have a cat and find that your allergy is a problem. The good news is that cats and people with allergies can live together! There's a lot you can do to make your life easier if you have allergies and a cat.
Allergies to cats are caused by a reaction to certain proteins found primarily in secretions from a cat's skin and in a cat's saliva. These proteins stick to your cat's hair and her shed skin.
Some people report developing immunity or growing out of the allergy to their cat. While this is certainly possible, don't depend on it. It is also possible that the allergic reaction will get worse with more exposure.
If you are getting a new cat and have concerns about allergies, consider a short-haired breed over a long-haired as they release less hair into your home environment. If you are interested in a purebred, consider a Devon or Cornish Rex. These cats lack some of the layers of hair found on other breeds and so may produce less reaction. The Sphinx breed is entirely hairless and extremely affectionate. Keep in mind that all of these cats groom themselves and an allergic reaction is caused by saliva just as much as by hair.
Once you have a cat, diligence around the house is key to limiting allergies:
Wipe down smooth surfaces in the home regularly and vacuum frequently as well
Frequently wash any bedding that your cat sleeps on
You may want to restrict your cat's access to certain areas of the house. The allergic person's bedroom is a definite cat no-go zone
Rooms with hardwood floors will retain less allergens and be easier to clean than carpet
If you have only a few rooms in your house with carpet, you probably should keep your cat out of those
Upholstered furniture pieces will retain a lot of allergens so you may choose to keep cats off of them or out of rooms that contain them
Providing the best cat care includes weekly brushing. It will be incredibly helpful in reducing allergic reactions because it helps prevent loose hair from getting into the air. Be particularly careful to groom in the springtime when your cat will be shedding its winter coat. Also consider buying Science Diet™ cat food specifically formulated to reduce shedding. Cleaning the litter box regularly will also help because the proteins that cause a reaction in saliva, hair and dander are found in urine as well. Whenever possible, all grooming should be done by someone who isn't allergic to cats. It should also be done outdoors if possible.
Talk to your doctor about what anti-allergenic drugs you can take to make your life easier and other possible ways to manage the problem.