====== 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.

<WRAP center round important 90%>
Allergy symptoms begin with histamine…  
but the disease is driven by the immune system that follows it.
</WRAP>

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===== Watch the Lecture =====

<WRAP lecturevideo>
{{youtube>VIDEO_ID_HERE}}
</WRAP>

<WRAP center round important 60%>
📄 **Download Slides (PDF)** \\ 
{{:office_hours:inflammation:episode_3_histamine.pdf|Download Slides}}
</WRAP>

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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.

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===== 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

<WRAP center round box 90%>
This is why patients say:  
“I touched it and immediately reacted.”
</WRAP>

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===== What Histamine Actually Does =====

^ Effect ^ Clinical Symptom ^
| Vasodilation | Redness |
| Increased permeability | Swelling |
| Nerve stimulation | Itching |
| Bronchoconstriction | Wheezing |
| Mucus secretion | Runny nose |

Blocked by:

  * [[allergy:drug_classes:first_generation_h1|First-Generation Antihistamines]]
  * [[allergy:drug_classes:second_generation_h1|Second-Generation Antihistamines]]

Important:

<WRAP center round tip 90%>
Antihistamines treat symptoms — not the immune reaction.
</WRAP>

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===== 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.

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===== 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:

<WRAP center round important 90%>
If antihistamines fail — the disease has moved beyond histamine.
</WRAP>

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===== Key Takeaway =====

Allergy is not a histamine disorder.

Histamine is the alarm bell.  
The immune system is the fire.

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===== Next Episode =====

→ [[office_hours:inflammation:episode_4|Episode 4 — Cytokines & Chronic Inflammation]]

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===== Related =====

  * [[office_hours:inflammation:start|Inflammation Series Home]]
  * [[allergy:start|Allergy Pharmacology]]
  * [[respiratory:asthma:start|Asthma Pharmacology]]
  * [[immunology:biologics:start|Biologic Immunomodulators]]
  * [[lectures:start|Full Lecture Series]]