There are 8 allergens associated with cats, this includes their hair, dander (which is flakes of dry skin), urine and saliva.
Fel d 1 is the most common allergen people react to. Most animal allergies are caused by lipocalin proteins, but cat allergy is different in that the main allergenic protein is a uteroglobin. Uteroglobin proteins are anti-inflammatory proteins produced by mucosal cells in most of the organs. This means Fel d 1 is found in cat saliva, urine and dander and in higher concentrations in male unneutered cats. As such there is no such thing as a hypo-allergenic cat (not yet), as hair is not the main culprit in this allergy.
Fel d 4 and Fel d 7 are lipocalin proteins, in other animal dander allergies this is the protein that most individuals react to. These proteins transport molecules like lipids and steroids around the body.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Cat.php
The allergy resources page for cat allergy covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with cat allergy .
There is also regularly updated links to useful websites specifically for cat allergies and the latest research papers on the topic.
If you have multiple allergies and are interested in how they are linked or want to find out more about food allergies outside the usual top 14 then you can follow me on Facebook or Twitter, where I share a food allergy card every day of the week.