Natalie is allergic to cats (and dogs, and cow's milk and eggs lol

). We noticed it around 5 months, at the same time her eczema and nasal congestion showed up!
For her, she breaks out in hives/rash within minutes of being in contact with our cat (and anywhere else she's rubbed with her hands). Generally she's ok just patting the cat, but if the cat has been cleaning herself, or if Natalie gets licked directly, the hives are there immediately. If it's really bad we give her a dose of antihistamine, but generally just washing the area with soapy water is enough for the hives to subside within 30mins or so. Her specialist actually advised keeping the cat, as animal allergies lessen with exposure (whereas food allergies increase with exposure...crazy but true!)

And being exposed to cats as children can apparently reduce the risk of asthma later in life (although my SIL would beg to differ!)
The good thing is that with Natalie's last blood tests, the cat allergy had come down from highly allergic to moderate, and we're definitely noticing less reactions (and the cat is retty much her best friend, so she spends all day with her). Not sure how much the allergic rhinitis is related to the cat allergy, but given that the snottiness disappeared by taking dairy and egg out of our diets, and she's still in contact with the cat, I'm guessing the hives are pretty much the only symptom.
Hope that helps a little.