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

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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

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