Allergic Reaction, Anaphylaxis, and Toxicologic Emergencies

1. What an Allergic Reaction Actually Is

Your immune system is built to attack things that don't belong in your body. So it flags a protein, decides it's a threat, and mounts a response. In an allergic reaction, that protein is harmless — peanut, penicillin, bee venom. The threat is fake, but the response is real.

Here's the mechanism. Cells called mast cells sit in your skin and airway, loaded with chemicals. When your immune system recognizes the allergen, it triggers those mast cells to dump their contents. The main one is histamine. Histamine does two things: it relaxes the walls of your small blood vessels, and it makes those vessel walls leaky.

Relaxed vessels widen. That's vasodilation, and it's the same effect you learn with the Chinese finger trap — pull against it and it tightens; stop pulling and it lets go. Histamine tells the vessels to let go. Leaky vessels let fluid escape into the surrounding tissue. That's why you get swelling. And when enough vessels do this throughout the body, you lose blood pressure. There's still the same amount of blood. But the container it's in just got bigger and started leaking.

So one chemical, two problems: swelling wherever the vessels are leaking, and a falling blood pressure wherever they're dilating. Everything you see in this lesson traces back to that.