Inflammation — Episode 3: Histamine & Immediate Hypersensitivity
This episode explains why allergies happen in seconds — and why antihistamines only partially fix the problem.
Histamine is not the whole allergic reaction.
It is only the opening signal.
Allergy symptoms begin with histamine… but the disease is driven by the immune system that follows it.
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Step 1 — Sensitization (First Exposure)
The first exposure does NOT cause symptoms.
Instead:
- Allergen presented to T-helper cells
- Th2 differentiation occurs
- B-cells produce IgE antibodies
- IgE binds mast cells
Patient is now primed
No reaction yet — but ready to react explosively next time.
Step 2 — Re-Exposure (The Actual Allergy)
The allergen binds adjacent IgE antibodies on mast cells.
→ Cross-linking occurs
→ Mast cell degranulates
→ Histamine released within seconds
This is why patients say: “I touched it and immediately reacted.”
What Histamine Actually Does
| Effect | Clinical Symptom |
|---|---|
| Vasodilation | Redness |
| Increased permeability | Swelling |
| Nerve stimulation | Itching |
| Bronchoconstriction | Wheezing |
| Mucus secretion | Runny nose |
Blocked by:
Important:
Antihistamines treat symptoms — not the immune reaction.
Why Symptoms Continue After Antihistamines
Histamine is only the early phase reaction
Within hours:
- Cytokines released
- Eosinophils recruited
- Leukotrienes produced
- Tissue inflammation develops
This is the late-phase reaction
Now antihistamines are weak.
Steroids work better because they block immune signaling.
Clinical Interpretation
| Disease | Best Therapy |
|---|---|
| Mild seasonal allergies | Antihistamines |
| Persistent allergic rhinitis | Intranasal steroids |
| Allergic asthma | Leukotriene blockers or ICS |
| Anaphylaxis | Epinephrine |
Patients often misunderstand this:
If antihistamines fail — the disease has moved beyond histamine.
Key Takeaway
Allergy is not a histamine disorder.
Histamine is the alarm bell. The immune system is the fire.