Let's talk about allergies. I'm sure you know someone who claims to have hay fever. That's an allergy, right? Or maybe someone with lactose intolerance. Fructose intolerance, maybe? What about the guy with iodine allergy? Gluten allergy? Penicillin allergy? Allergy to anesthetics at the dentist? One time I met someone who claimed to have sugar allergy. Can that be?
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So, first of all, let's clear out what an allergy actually is. An allergy is a type of hypersensitivity reaction. Hypersensitivity reactions are undesirable reactions produced by the normal (though overactive) immune system. They are usually exaggerated responses to something that is not really harmful but is identified as such. These reactions may range from damaging to deadly. There are four distinct types of hypersensitivity reactions and "allergy" is the alternative name for the type I.
So why am I taking my time to explain you this? - you may ask... Well, for the same reason I would explain the boy who cried wolf what wolves are actually capable of. Thing is, allergies can be deadly. If I suspect someone to have some allergy, I would keep that substance far away from them. If I really must give them that substance, I will give them drugs to treat the allergy even before they get in contact with it (that's called prophylaxis).
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Is the guy above really allergic to lactose? No... Lactose is a sugar. All mammals digest it as babies by secreting lactase (an enzyme) to break it down. Humans (especially in europe) evolved the capability to preserve that enzyme into adulthood. Not all of us though. Some still stop producing it when they grow up, and those are the ones who are said to be "lactose intolerant" because if not digested timely in the gut, bacteria there will do it instead. When doing it, bacteria produce gas, though, which can be uncomfortable and cause other problems. Similarity with allergies: 0
Is the other guy above allergic to iodine? Well, no... Iodine is everywhere... Our bodies need iodine to survive. Without it, you are kinda dead. So, do you really believe someone can have its immune system permanently attacking an essential-to-life element and still be fine? Plus, how can an element be an antigen? Makes no sense. Iodine allergy doesn't exist. Still, you see everywhere in hospitals people who are labeled as "allergic to iodine", because they say they are and... well... allergies are serious things and doctors WILL document them even if they make little sense. Now, what does exist is people who can't receive massive doses of iodine at once because they have sick thyroid glands. Or people who can't receive iodinated contrast because they have sick kidneys. Or people who actually have a real allergy to iodinated contrast (not to the iodine itself though!). So, as an example, don't say you are allergic to iodine if you simply have a sick thyroid, otherwise you will receive a whole bunch of prophylactic (preventive) drugs if you ever really need CT contrast. Or maybe you won't even get any contrast if it's not absolutely needed (but would get you a better diagnosis). Try to understand what you really have, it's also your responsibility and not only your doctor's.
Now, penicillin allergy... They do exist! But many people who say they have it actually have none. You can't really take a penicillin shot, experience some intestinal discomfort and start telling the story around that you are allergic to penicillin. Intestinal discomfort is a side effect and just that! If you tell a doctor you are allergic to penicillin, he will believe you! Even if the story doesn't really sound like an allergy, he will document it (you just can't risk it, legally especially). You are the one who loses for being so careless with your self-diagnosis, as you will receive a different antibiotic "just to be safe", even if it won't be the most appropriate.
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Want a more dramatic case? Here it goes: "Allergy to local anesthetics" after someone had a seizure followed by a cardiac arrest at the dentist? Well, that sounds just like local anesthetics systemic toxicity (LAST) and has absolutely nothing to do with the immune system. Too much anesthetic somehow got into his bloodstream too fast, causing a direct toxic effect on his brain (seizure) and heart (arrest). Did the dentist aspirate before he injected to be sure the tip of the needle was not in a vein? Either way, that person should not be doomed to never receiving local anesthetics ever again. I'm not saying they don't exist nor am I saying the outcome couldn't have been tragic but that reaction to local anesthetics was NOT an allergy and is unlikely to reoccur in the future if the drug is properly applied.
You know who else loses with all this? Every single person who is actually really allergic to something, and may be discredited and put in danger because of all the people above (for example by a restaurant's chef who, after hearing 438 people saying they are allergic to "thick slices of mushrooms" and similar nonsense, gets a bit too numb to your deadly peanut allergy which will kill you on the spot if you put your lips on even a badly washed spoon which was two days ago in brief contact with peanut butter). People, be careful with your words and your self-diagnosis. When in doubt, use the word "intolerance" instead.
PS: the hay fever above actually IS an allergic reaction.
PS2: the guy above who claimed to have "sugar allergy" actually had diabetes! True story...