8 Gallucci - Cell-Mediated Allergy (Type IV)

Obj. 1

Summarize general mechanism of Type IV hypersensitivity.

📌 T cell–mediated (NOT antibody-mediated)

🪜Step-by-step:

1. Antigen is presented to T cells (CD4⁺ or CD8⁺)

2. T cells become activated

3. Release cytokines

4. Recruit macrophages + inflammatory cells

5. → Tissue damage

🔑 Key point:

• NO antibodies involved

• This is cell-mediated immunity

Obj. 2

Relate how inflammation is involved in the Type IV response

📌 Inflammation is the main cause of damage

• T cells → cytokines

• Cytokines → recruit macrophages

• Macrophages → cause tissue destruction

💡 Important:

👉 Damage is from the immune response, not the antigen itself

Obj. 3

Describe at least 1 common disease that is a Type IV reaction.

🏥 Common Type IV Disease

✅ Most common:

👉 Allergic Contact Dermatitis

📌 Examples:

• Poison ivy

• Poison oak

• Occupational skin reactions

Obj. 4

Describe the most common Type IV allergens.

📌

• Small, lipid-soluble molecules (haptens)

• Must penetrate skin

👉 Examples:

• Plant allergens (poison ivy)

• Chemicals

• Metals (e.g., nickel)

Obj. 5

Recognize and describe symptoms of a skin Type IV reaction and suggest basic therapy

🤒 Skin Type IV Reaction (Symptoms + Therapy)

➡ Symptoms

• Redness

• Itching

• Rash

• Swelling

• Delayed onset (24–72 hours)

➡ Basic Therapy

• Avoid allergen

• Anti-inflammatory treatment (e.g., steroids)

⏱Key Feature

❗ Delayed response

• Starts 8–12 hours

• Peaks 48–72 hours

Obj. 6

Describe what Extrinsic Allergic Alveolitis is and its relationship to Type III Type IV reactions.

🌾 Extrinsic Allergic Alveolitis

👉 AKA:

• Hypersensitivity pneumonitis

📌 What it is:

• Lung reaction to organic dusts (occupational exposure)

🔗 Relationship to Type III & Type IV:

• Early phase → Type III (immune complex)

• Later phase → Type IV (cell-mediated)

🔥Summary

• Type IV = T cells (NO antibodies)

• Delayed (48–72 hrs)

• Inflammation = damage

• Most common = contact dermatitis

• Alveolitis = Type III + Type IV combo

⭐ 1–2 Sentence Exam Answer

👉 Type IV hypersensitivity is a T cell–mediated immune response in which cytokine release recruits macrophages and causes inflammation and tissue damage, with common manifestations such as contact dermatitis and delayed reactions, and conditions like extrinsic allergic alveolitis involving both Type III and Type IV mechanisms.