Overview
This lecture explains the four types of hypersensitivity reactions, highlighting mechanisms, key antibodies, timelines, and relevant clinical examples.
Type I Hypersensitivity (Anaphylactic)
- Type I reactions are immediate allergic responses, often occurring within minutes of exposure.
- IgE antibodies bind to mast cells or basophils, causing degranulation and release of inflammatory mediators, mainly histamine.
- Histamine increases blood vessel permeability, causes swelling, redness, mucus production, and smooth muscle contraction (e.g., difficulty breathing).
- Antihistamines help mild reactions; epinephrine is needed for severe anaphylactic shock.
- Systemic anaphylaxis is life-threatening, often triggered by bee stings, foods (e.g., peanuts), or drugs (e.g., penicillin).
- Localized reactions include asthma, hay fever, and hives.
Type II Hypersensitivity (Cytotoxic)
- Type II reactions occur within 5–8 hours.
- Involves IgM and IgG antibodies, which activate complement to destroy target cells.
- Classic examples: transfusion reactions and hemolytic disease of the newborn.
Type III Hypersensitivity (Immune Complex)
- Type III reactions form antigen-antibody (mainly IgG) complexes in blood serum.
- These complexes trigger inflammation and can damage organs (e.g., kidneys).
- Reaction occurs in response to soluble antigens, not cell-associated ones.
Type IV Hypersensitivity (Delayed/Cell-Mediated)
- Type IV reactions occur 24–48 hours after antigen exposure.
- Not antibody-mediated; involves T cells and other immune cells.
- Examples: transplant rejection, contact dermatitis (e.g., poison ivy, latex allergy), and tuberculin skin test reactions.
Key Terms & Definitions
- Anaphylaxis — Severe, rapid allergic reaction that can be life-threatening.
- IgE — Antibody associated with allergic (type I) reactions.
- Mast cells/Basophils — White blood cells releasing histamine during allergic responses.
- Degranulation — Release of substances from immune cells, initiating inflammation.
- Histamine — Molecule causing vessel permeability, swelling, and mucus in allergies.
- Complement — Group of proteins that help antibodies destroy pathogens.
- Immune Complex — Cluster of antigens and antibodies causing type III reaction.
- Contact Dermatitis — Skin inflammation caused by direct contact with allergens.
Action Items / Next Steps
- Review examples of each hypersensitivity type.
- Practice distinguishing between hypersensitivity mechanisms.
- Prepare for questions about clinical scenarios involving allergic and immune responses.