Hypersensitivity reactions are exaggerated or inappropriate immune responses that result in tissue damage. They are classified into four main types based on their immunological mechanisms and clinical manifestations:

⚡ Type I: Immediate (Anaphylactic)

Mediators: IgE, Mast Cells, Histamine, Leukotrienes
Timing: Minutes after allergen exposure
Mechanism: Allergen cross-links IgE on mast cells → release of histamine and vasoactive mediators → smooth muscle contraction, vasodilation, increased permeability.
Clinical Examples: Allergic asthma, hay fever 🤧, food allergies 🥜, anaphylaxis 🚑
Clinical Importance: Life-threatening anaphylaxis requires immediate adrenaline (epinephrine) injection.

🩸 Type II: Cytotoxic (Antibody-Mediated)

Mediators: IgG, IgM, Complement, NK cells
Timing: Hours to days
Mechanism: Antibodies bind to antigens on host cells → complement activation or ADCC (antibody-dependent cellular cytotoxicity) → cell lysis.
Clinical Examples: Autoimmune haemolytic anaemia, Goodpasture’s syndrome, transfusion reaction 💉, rheumatic fever
Clinical Importance: Explains transfusion mismatches and autoimmune cytopenias.

💥 Type III: Immune Complex-Mediated

Mediators: Antigen–antibody (Ag-Ab) complexes, Complement, Neutrophils
Timing: Hours to weeks after exposure
Mechanism: Immune complexes deposit in tissues → complement activation → neutrophil recruitment → inflammation and tissue damage.
Clinical Examples: Systemic lupus erythematosus (SLE) 🦋, rheumatoid arthritis, post-streptococcal glomerulonephritis
Clinical Importance: Leads to vasculitis, nephritis, arthritis - key mechanism in autoimmune disease flares.

🕒 Type IV: Delayed-Type (T-cell Mediated)

Mediators: CD4+ and CD8+ T cells, Cytokines
Timing: 24–72 hours after antigen exposure
Mechanism: Sensitized T cells release cytokines → macrophage activation → tissue injury and granuloma formation.
Clinical Examples: Contact dermatitis (nickel allergy 💍, poison ivy 🌿), tuberculin skin test (Mantoux), chronic graft-versus-host disease
Clinical Importance: Central in transplant rejection and granulomatous inflammation (e.g., TB).

📊 Quick Comparison Table

Type	Key Mediators	Timing	Mechanism	Examples
⚡ I (Immediate)	IgE, Mast cells	Minutes	Degranulation → histamine release	Anaphylaxis, hay fever, asthma
🩸 II (Cytotoxic)	IgG, IgM, Complement	Hours–days	Antibodies bind to host cells → lysis	AIHA, Goodpasture’s, transfusion reaction
💥 III (Immune Complex)	Ag-Ab complexes, Complement	Hours–weeks	Complex deposition → inflammation	SLE, RA, PSGN
🕒 IV (Delayed)	T cells, Cytokines	24–72 hrs	T-cell mediated injury	TB skin test, contact dermatitis

The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms.

Types of Hypersensitivity Reactions

⚡ Type I: Immediate (Anaphylactic)

🩸 Type II: Cytotoxic (Antibody-Mediated)

💥 Type III: Immune Complex-Mediated

🕒 Type IV: Delayed-Type (T-cell Mediated)

📊 Quick Comparison Table

Categories