The dander from the cat is the main culprit, though the urine
, and even vomit can be allergenic.
Don't let the cat near your bedroom. If the cat was on your bed in the past, vacuum the matress, boil the sheets, dry clean the covers, vacuum the carpet. You may have to do this several times to get rid of the dander. Using salione or steroid sprays at nite might help.