My little Marigold (a pomeranian) had an adverse reaction to a shot a few months ago, so I know how bad you both feel.
The good news is, off the bat, that the worst reactions (anaphlaxis or shock) occur most often very soon after injection -- so kitty is *probably* not at significant risk of serious problems by this stage.
I'm not sure there are any home remedies, unfortunately. Might be a good idea to call vet tomorrow and see if they can do anything, but I don't think so. What they can do is either adjust the vaccine for next time (use a killed vaccine rather than a live one) or in some cases can give a medicine that prevents symptoms from bad reaction from getting too bad.
The most serious reaction is anaphlaxis -- shock, which can cause respiratory failure. It is a life threatening emergency. If kitty seems to be doing badly, and you fear the onset of serious symptoms, you might want to take her to an emergency clinic as a precaution. (I tend to be over-prepared about everything, but what I would do NOW if I were you even if the cat seems to be okay is to at least identify the nearest emergency clinic and have a plan about how to get there in a hurry. Hopefully you won't need to go, but iuf you do, it's great to not have to figure it out when you are in panic with a seriously ill animal.)
Here a little reading for you.
TYPES OF REACTIONS
* Mild. Mild reactions include fever, sluggishness and loss of appetite. Mild reactions usually resolve without treatment.
* Moderate. Urticaria is a moderate vascular reaction of the skin marked by hives or wheals and rapid swelling and redness of the lips, around the eyes, and in the neck region. It is usually extremely itchy. Urticaria may progress to anaphylaxis, which is considered life-threatening. Urticaria is the most common reaction in cats.
* Severe. The most severe reaction is anaphylaxis, a sudden, severe allergic response that produces breathing difficulties, collapse and possible death. Symptoms usually include sudden onset of vomiting, diarrhea, staggering, rapid drop in blood pressure, swelling of the larynx leading to airway obstruction (and inability to breathe), seizures and cardiovascular collapse or death. This reaction is life-threatening for your cat.
Both anaphylaxis and urticaria are reactions that are triggered by antibodies that the immune system has made to some portion of the vaccine and usually requires at least one previous exposure to the vaccine. The antibodies cause inflammatory cells like basophils and mast cells to release substances that cause the allergic reaction. The impact on the cat may be life threatening but if treated successfully the prognosis for long-term health is good. Mild reactions usually resolve without treatment.
Veterinary Care
Diagnostic tests
There is no diagnostic test for anaphylaxis or urticaria, but a quick physical exam will show common signs of an allergic reaction and the history will reveal recent vaccination.
Treatment
* Anaphylaxis is an extreme emergency. Your veterinarian will begin immediate emergency life support including establishing an open airway, oxygen administration, intravenous fluids to increase blood pressure and drugs such as epinephrine, diphenhydramine and corticosteroids. Cats that survive the first few minutes usually return to normal health. Anaphylaxis usually occurs soon after vaccination, often while the cat is still in the veterinary clinic.
* Urticaria occurs soon after vaccination, often shortly after the animal gets home. Your veterinarian will recommend immediate return to the hospital for treatment. An oral antihistamine such as diphenhydramine (Benadryl®) may be recommended to initiate treatment. Urticaria is usually treated successfully with injectable corticosteroids like dexamethasone or prednisone. Antihistamines do little to help with acute allergic reactions but may be given by injection to help prevent recurrence of symptoms after steroids wear off.
* Mild vaccination reactions usually require no treatment. However, if the symptoms persist for more than 24 hours, call your veterinarian.
Home Care
Be sure to schedule vaccination appointments when you will be available to monitor your cat after the vaccine is administered. Be sure to call your veterinarian with any questions or concerns.
Preventative Care
The good news is that severe vaccination reactions are rare. The risk of anaphylaxis and urticaria are much more rare than the benefit of the vaccine in most cases. You can limit vaccines to those that prevent diseases to which your cat may be exposed. Your veterinarian is the best judge of what vaccines are needed to protect against the diseases in your area.
Your veterinarian will record any adverse reactions to vaccines to help prevent those vaccines from being administered again. It is a good idea to also keep a record yourself.
http://petplace.netscape.com/articles/artShow.asp?artID=619