Allergies

I have metal allergies. They cause a skin reaction only. Benadryl, topically and orally helps. Docs have never suggested shots for this. I'm told to avoid the metals in question or provide a barrier to contact. For example, earrings and belt buckles can be coated with clear nail polish. The metal rivet on the back of a jeans button can be covered with tape. A pocket watch can be substituted for a wrist watch.

I'm allergic to whatever is in eyeshadow that makes it glittery (not sure if it's mica or what). Definitely the contact issue of itchiness and redness but because it's on the eye, I also get swollen eyelids and seriously itchy eyes similar to a pollen-allergy reaction. I have allergy eyedrops to quickly stop the itching. I suppose in a worst-case scenario, the mica could start a chain-reaction of allergy symptoms if the immune system kicks in (especially if near the eye or breathed in somehow), possibly leading to an asthmatic reaction similar to an extreme seasonal allergy. Also, I saw this on Web MD:

There are two types of skin reactions to beauty products. One, called irritant contact dermatitis, happens when something actually damages your skin. Your skin might burn, sting, itch, or get red right where you used the product. You might get blisters and have oozing, especially if you scratch.

The other kind of reaction actually involves your immune system. It’s called allergic contact dermatitis and symptoms include redness, swelling, itching, and hives. Your skin can get red and raw. You can get an allergic reaction on any part of your body, although it happens most often on the face, lips, eyes, ears, and neck.

Here's the link to the general allergy page on Web MD. You can get more info on symptoms and treatments (see the nickel allergy sub-page for an example of metal allergy).
http://www.webmd.com/allergies/guide/default.htm

I'm allergic to something (probably nickel, but I'm not sure). I get an itchy rash if I wear something with whatever in allergic to in it. Sometimes blisters. No specific treatment except to avoid the metal, do the barrier thing and use OTC allergy stuff. Reactions definitely can happen within hours, sooner than that for me.

One of my kids has a severe food allergy. I'm not sure if the same thing could happen with the allergens you've mentioned, but what could happen (and has happened here--very scary!) is anaphylaxis, which included tongue swelling, hives, then skin all over the body turned red like a very bad sunburn, ambulance ride to the hospital and a shot--though EMT said they usually don't give a shot in the ambulance--instead wait for the doctors to do this at hospital, more shots at hospital ER, admitted for monitoring overnight. (Hives started on the way to the hospital, overall redness happened after were at the hospital for a little while--can't remember how long). Now needs to stay away from that food, carry an Epi-pen at all times, and doctor said to wear one of those bracelets that says what one is allergic to. Symptoms (started with tongue swelling) came on very fast--probably less than a minute. You might want to research "anaphylaxis" for more information. I suppose you'll need to determine if your character will have a mild, moderate, or severe allergic reaction.

After our trip to the hospital, we had to go get blood drawn so that the blood could be tested to determine what caused the allergy. Then we had to visit our regular pediatrician (same day as discharge from hospital), who referred to an allergist for skin tests. (The blood test didn't test for as many things as the skin test.) We had to wait a week or two for an allergist appointment. For that, prescription numbing cream was applied an hour or so before the appointment (can't remember exactly how long) to the arm (I think it can also be done on the back). At the doctor's office, around 20 different allergens were applied to the skin--just tiny amounts, and numbered with ink (nurse wrote numbers on the skin with a pen) to coordinate with the numbered listing of allergens on a paper. After around 20 minutes (can't remember exactly) the doctor came in to see which substances caused reactions--looked like mosquito bites. There are far more allergens that the allergist could have tested for, but we'd narrowed it down.

In addition to the severe food allergy, we also have mild seasonal allergies here, for which the allergist recommended over-the-counter medication. He didn't mention anything about allergy shots, and no need for follow-up visits with the allergist.

Hope this helps with your story.