system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does allergy medicine work? Does it suppress the immune system in some way or does it use another mechanism?
gpt
Antihistamines (Benadryl/diphenhydramine, Zyrtec/cetirizine) block the effect of a pretty specific part of the immune system, the histamine released by mast cells and basophils in response to allergens. Mast cell stabilizers like cromolyn prevent release of histamine to begin with. These are sort of technically suppressing a little bit of immune function, but it's a part of the immune system that mostly defends against parasitic worms, not bacteria or viruses. Leukotriene receptor blockers like montelukast stop some of the signals that make airways clamp down in asthma.

Corticosteroids definitely suppress immune function in a broad way, though how much they suppress depends on where they go. Nasal steroids like fluticasone are targeted narrowly and have few side effects, while a steroid like prednisone taken by mouth goes everywhere and can do pretty much anything.