An allergy is when our immune system mistakes a harmless substance for a germ, and begins to fight it. All the symptoms we get are caused by our own defenses at work. Nobody knows why people are allergic, most likely a genetic tendency.
You can’t have an allergy the very first time a substance gets into you, because the body has to set up its immune system to recognize it. But in real life, substances wind up in us without our realizing it, like penicillin that’s put in animal feed to promote growth. On the other hand, an allergic person might take penicillin 100 times without a problem, and become allergic the 101st.
Allergies can manifest themselves in different ways (see diseases with links for more explanation):
1. Respiratory allergies, from inhaling plant pollen, dust mites (microscopic insects that live in dust), animal dander (skin flakes from pets), microscopic fragments from cockroaches, etc. (sorry if this sounds gross). The illnesses they cause are:
- Allergic Rhinitis (Hay Fever) — runny nose, lots of sneezing, maybe itchy eyes
- Asthma — swelling and spasm inside the bronchi of the lungs
2. Eye Allergies:
- Allergic Conjunctivitis — teary, itchy eyes, from the same substances that cause respiratory allergies
- Contact Allergy — from substances like make-up that get into the eye
3. Hives (medical term is “Urticaria”) — Wheals on the skin that come, go, move around, and always itch. They’re caused by something ingested, the most common substances being medicines, shellfish (shrimp, etc.), nuts, soy, and avocado, but almost anything can do it. Some people can get hives from pressure on the skin, or from the cold. If a rash does not itch, it’s not going to be a true allergy (though sometimes medicines can cause non-itchy rashes by other means). People who get hives do not go on to get life-threatening allergies, and do not need to carry Epi-Pens® (device to self-inject adrenalin, an antidote)
4. Angioedema — This is a strange term that refers to swelling of the lips, eye lids, face, tongue, and/or throat. It’s caused by the same substances that cause hives (see above). Angioedema can be dangerous, because swelling inside the throat can be fatal in a few minutes. Anyone who’s definitely had angioedema should carry an Epi-Pen®.
5. Anaphylaxis — When an allergy causes sudden spasm of the lung’s bronchi, like what happens in asthma, or causes Shock. Anaphylaxis can be rapidly fatal. It’s usually caused by something injected, like medications, or bee stings, but can also happen to people extremely allergic to certain foods, especially peanuts. Anyone who’s definitely had anaphylaxis should carry an Epi-Pen®. See also Improvised Treatment in the Field.
6. Contact Allergies (Contact Dermatitis) — An allergy to something that touches the skin. Nickel in necklaces is a famous cause, latex in rubber is another, and also poison ivy / poison oak, but many substances can do it. Contact dermatitis causes very itchy blisters.
7. Eczema — The medical term is Atopic Dermatitis; this is a genetic condition in which a person is sort of allergic to themselves. They get itchy red patches on the skin, often with scales or tiny blisters. Most common sites are creases in the elbows, behind the knees, or on the neck, but it can happen anywhere. People with eczema may often tend to have asthma or allergic rhinitis, & vice versa.
Allergic to Medication — We obviously can’t give a medicine to someone who’s allergic to it. Allergy here means having had Hives (Urticaria), Angioedema, or Anaphylaxis from it. It does not refer to vomiting, dizziness, or any other side effects.
Studies show that the vast majority of people who say they’re “allergic to penicillin” really aren’t. Many had some other side effect, or maybe their mother told them from some childhood event that nobody remembers any more. Usually we’re able to find another antibiotic to use instead, but sometimes not. For example, when syphilis occurs in the brain, or during pregnancy, we absolutely must give penicillin. So we sent the patient to the ICU, give them the tiniest dose with life-saving treatment immediately available, then increase the dose every minute or two, to teach the immune system to get used to it.
Another case is when a person gets a shot of a medication, and immediately collapses. It could be life-threatening shock from anaphylaxis, but that usually takes longer from injections into muscle. More likely, it’s what we call a vaso-vagal reaction — a strong emotional response that makes someone faint (like if they see blood). That’s harmless, and not an allergy. A key to diagnosis is counting the heart rate (taking a pulse). Shock causes rapid heart rates; vaso-vagal reactions cause very slow ones.
If you think you had a mild allergy to an antibiotic, i.e. Hives (not Angioedema, not full-blown Anaphylaxis), there’s a way to find out by “Oral Challenge,” which has been shown to be safe. Ask your healthcare provider to give you an oral dose, then wait around the office for 2-3 hours. If no reaction happens, you’re not allergic! If you do get a reaction, well, you are allergic; & there in the office they can give you treatment right away.