Eli5: How do antihistamines work and what is the difference between first generation and second?
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The window screen analogy isn’t quite right. I describe them to patients as the equivalent of paper towels.
When your allergy cells are exposed to something you are allergic to (pollen, dander, food, etc) they react by exploding and releasing a bunch of stuff including a molecule called histamine.
Histamine then binds to a receptor on other cells and causes the itchy, red, raised hives or the itchy, watery eyes or the itchy, runny, sneezy, nose or your lungs to tighten up depending on where it is released.
Antihistamines are like paper towels in that they ‘mop up’ the histamine that is released. They can’t block pollen nor the reaction of your allergy cells. All they can do is block (actually compete for the binding spot) on the receptor and then block all the symptoms.
First generation easily crossed the blood brain barrier. They make you sleepy and are shorter acting. They suck and should really never be used these days.
Second generation were made to last longer and not cross the blood brain barrier as much. Claritin was the first so got early brand name recognition but it’s too weak to do much. Zyrtec is great and cheap but makes 10% of people sleepy. Allegra is nearly as strong as Zyrtec and will not make you sleepy.
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