In this lecture, we’ll discuss allergic reactions including anaphylaxis, urticaria, angioedema and food allergy. So there are generally 4 types of hypersensitivity reactions. This is your basic science material. Type 1 is IgE mediated and an example would be anaphylaxis. Type 2 is IgG or IgM mediated antigen on a target cell and an example of that would be rheumatic heart disease. Type 3 is when there are circulating antigen/antibody complexes. An example of this type of reaction is for example serum sickness. And a type 4 hypersensitive reaction is generally a T-cell mediated response such as poison ivy, or a PPD test, or milk protein allergy presenting as blood in the stool as an infant. So, let’s start with type 1 hypersensitivity reactions. In general, these require exposure to a protein that’s large enough to be an antigen. So, the patient has an initial exposure to that antigen, and they produce an antigen-specific IgE. That IgE now binds to a mast cell, and on a subsequent exposure to that antigen, they generally have a generalized mass cell degranulation resulting in a type 1 hypersensitivity reaction. There are many things that can trigger such a reaction. These include foods, especially food additives, legumes, tree nuts, seafood, eggs, dairy, shellfish, berries. These are all very common. In fact, among children who have type 1 hypersensitivity reactions to food about 85% of them are to food when we know what the allergen is. Still, remember, at least a third of the time we have no idea what triggers the allergy. Also, patients can be allergic to medications. Probably penicillins and sulfas are the most common, but they could also be allergic to salicylates to certain vaccines or to anaesthetics. Also patients can have environmental allergies. Bee stings and wasp stings are particularly problematic. Patients may be allergic to temperature changes to exercise, to latex. Lots of different possibilities. So, urticaria is our classic type 1 hypersensitivity reaction. We also call it “hives”. These lesions will be anywhere from 1 mm to 10 cm large. They can be huge or small. These lesions are profoundly itchy, and they’re stimulated by skin contact, which means the more you scratch them, the more they itch. What you’ll notice about them is that they sort of “come and go” even before your eyes. You may walk in, see a patient with hives, and before you’re done with your conversation, the one you were looking at on the leg is gone, and there’s a new one on the arm. They may be associated with angioedema, which is another type of type 1 hypersensitivity reaction. What’s key is that these are acute and generally short-lived. Patients usually only have this reaction for maybe days to maybe a week out, from when they’re first encountered that antigen, and had a response to it, which would be the second time they ecountered the antigen. Alternatively, occasionally they can become chronic. These are when hives last longer than 6 weeks. This can really be problematic when it happens. So, the diagnosis of urticaria is based on appearance. This is a great picture. Just remember they’re sort of serpiginous, round with central clearing. This is a classic hives reaction. They’re a little bit raised. We’re going to treat this with oral antihistamines. That’s our mainstay of therapy, basically diphenhydramine. Patients may add H2 antagonists, such as ranitidine, to the mix to try and limit the effect of the histamines. This has a minimal benefit, but it’s certainly isn’t particularly harmful to be on ranitidine for a few days while you’re waiting for your hives to improve. What’s key is steroids are generally not helpful, so we do not routinely provide steroids to patients with urticaria. So, angioedema, I mentioned this earlier. This is sort of like hives gone wild. This is a general localized swelling that happens in an area, often on the face. It can happen around the eyes. It can happen on the lips, the mouth, or the tongue. It may happen on the genitalia, or the extremeties, or even in the bowel wall causing either bowel discomfort or nausea, vomiting, or more systemic symptoms. What’s key though and where we would worry, is when this involves the lips, the mouth, and the tongue, because we worry about the patency of the airway. Sometimes, the swelling can get so bad, the patient may have trouble breathing. This is an allergic reaction — or it can be inherited. There are rare diseases such hereditary angioedema where patients can get this randomly throughout their lives. In specific, we sometimes see this to certain medications and the classic one is the ACE inhibitor. Any patient on an ACE inhibitor, who gets this reaction, it’s an emergency because we may lose that airway. Additionally, NSAIDS can do this and calcium channel blockers can rarely do this. The classic one is ACE inhibitors. So, what do we if we have a patient with angioedema? Well, first, remove the offending agent. Get them off the drug that’s caused it or whatever has caused it, get them out of it, get it out of them. Then we’re going to treat it similarly to hives. So, we’re going to give the benadryl we talked about. We’ll probably do that around the clock. We may add Zantac. If the airway is involved, we generally would admit these patients to the hospital to make sure that that airway isn’t goint to become compromised. If the airway is obstructed, we immediately give epinephrine and provide immediate airway stabilization and airway support. It may require intubation and in very rare cases, an actual tracheostomy, if you’re incapable of getting an intubation. Sometimes a layringoscopic intubation is needed.