Q.While antacids and antiallergic drugs interfere with the function of histamines, why do these not interfere with the function of each other ?

Antihistamines: The Allergy Blockers

Imagine your body has a security system. When a harmless substance like pollen or dust enters, your immune system sometimes mistakes it for a dangerous invader. It sends out a chemical alarm — that alarm is histamine.

Histamine is a natural compound stored in mast cells (a type of white blood cell). When released, it binds to receptors on blood vessels and nerves, causing:

• Blood vessels to widen and leak fluid → swelling and redness
• Nerve endings to get irritated → itching and sneezing
• Mucus glands to overproduce → runny nose and watery eyes

This is the classic allergic response: the body overreacting to a false threat.

---

What Antihistamines Do

An antihistamine does not stop histamine from being released. Instead, it blocks the receptors (specifically H1 receptors) that histamine would normally bind to. Think of it like putting a dummy key in a lock — histamine has the key, but the lock is already jammed. The alarm still sounds, but no one can open the door.

---

Two Generations, Two Different Effects

The key difference is lipophilicity — how well the drug dissolves in fat. First-generation antihistamines are more fat-soluble, so they slip into the brain and cause drowsiness. Second-generation ones are designed to be less fat-soluble, so they stay in the bloodstream and block only peripheral H1 receptors. …