Hypersensitivity Reactions

Type I = IgE/mast cells (anaphylaxis). Type II = IgG against cells. Type III = immune complexes. Type IV = T-cell mediated (delayed).

Detailed Explanation

Type I - Immediate/Anaphylactic:

IgE-mediated

Mast cell/basophil degranulation

Histamine, leukotrienes release

Examples: Anaphylaxis, allergic rhinitis, asthma, food allergies

Occurs within minutes

Type II - Cytotoxic/Antibody-mediated:

IgG or IgM against cell surface antigens

Complement activation or ADCC

Examples:

- Autoimmune hemolytic anemia

- Transfusion reactions

- Goodpasture syndrome

- Rheumatic fever

- Myasthenia gravis

- Graves' disease

Type III - Immune Complex:

Antigen-antibody complexes deposit in tissues

Complement activation → inflammation

Examples:

- Serum sickness

- Lupus (SLE)

- Post-streptococcal glomerulonephritis

- Arthus reaction

Type IV - Delayed/Cell-mediated:

T-cell mediated (no antibodies)

Takes 24-72 hours

Examples:

- Contact dermatitis (poison ivy)

- TB skin test (PPD)

- Transplant rejection

- Type 1 DM (destruction of β cells)

Clinical Correlation

Anaphylaxis (Type I) requires immediate epinephrine. Contact dermatitis (Type IV) takes 24-48 hours to develop because T cells need time to be recruited and activated.

Memory Trick

"ACID: Anaphylactic, Cytotoxic, Immune complex, Delayed. Type 4 is the slow one - like waiting 4 days."