Comprehensive Guide to Types I-IV Hypersensitivity Reactions

Nomenclature & Classification: Also referred to as Immediate IgE-mediated hypersensitivity.
Memory Aid for Studying: Cat allergy.
Key Characteristics:
- Features an immediate immune reaction upon subsequent exposure to a specific antigen.
- Driven by antigens that stimulate antibody production.
Etiology:
- Genetic factors/predisposition.
- Exposure to an environmental or external antigen.
Key Immune Cells Involved:
- B cells
- IgE antibodies
- Mast cells (responsible for releasing histamine)
B-Cell Involvement: Yes.
Antigen Examples:
- Environmental antigens
- Pet dander
- Bee stings
Pathogenesis:
- Initial exposure causes B cells to synthesize IgE antibodies.
- IgE antibodies attach to the surface of mast cells.
- Upon subsequent exposure to the antigen, mast cells degranulate and dump histamine into surrounding tissues.
Diseases and Conditions:
- Asthma
- Urticaria
- Allergic rhinitis
Clinical Manifestations:
- Atopic Reactions: Stuffy nose, hives, and itching.
- Anaphylactic Reactions: Decreased blood pressure (decreased BP), increased heart rate (increased HR), and potentially death.

Nomenclature & Classification: Also known as Cytotoxic or "Cell death" hypersensitivity.
Memory Aid for Studying: Blood transfusion.
Key Characteristics:
- Direct targeting and destruction of host or target cells.
Etiology:
- Autoimmune attack.
- Infection.
Key Immune Cells Involved:
- IgG antibodies
- IgM antibodies (produced by B cells)
- White Blood Cells (WBCs / phagocytes)
- Complement system
B-Cell Involvement: Yes.
Antigen Examples:
- Transfusion with the wrong blood type.
- Autoimmune self-antigens (yourself).
Pathogenesis:
- Antibodies bind to target cells and cause them to clump together.
- Phagocytes and the complement system destroy the aggregated target cells.
Diseases and Conditions:
- Blood transfusion reaction
- Hemolytic anemia
Clinical Manifestations:
- Classic signs of a blood transfusion reaction, including fever, chills, headache, and systemic distress.

Nomenclature & Classification: Also known as Immune Complex Reaction.
Memory Aid for Studying: Carl Brown - RA (Rheumatoid Arthritis).
Key Characteristics:
- Antigen-antibody immune complexes form and deposit into host tissues.
- Inflammation triggered by complex deposition destroys surrounding tissues.
Etiology:
- Antigen exposure.
Key Immune Cells Involved:
- IgG antibodies
- IgM antibodies
- Complement system
- Neutrophils
- Mast cells
B-Cell Involvement: Yes.
Antigen Examples:
- Body's own self-antigens
- Inhaled antigens
- Bacteria or viruses
Pathogenesis:
- Circulating immune complexes form from antigen-antibody binding.
- These immune complexes deposit into specific tissue sites.
- Inflammatory response triggered by the complexes destroys host tissue.
Diseases and Conditions:
- Rheumatoid arthritis (RA)
- Glomerulonephritis
Clinical Manifestations:
- Manifestations depend directly on the tissue site where the immune complexes are deposited.

Nomenclature & Classification: Also known as Delayed hypersensitivity.
Memory Aid for Studying: Poison ivy, wedding ring.
Key Characteristics:
- Cell-mediated response with a delayed clinical onset after exposure.
Etiology:
- Antigen exposure.
Key Immune Cells Involved:
- T cells
- Cytokines
- Mast cells
- Macrophages
B-Cell Involvement: NO (the only hypersensitivity reaction type without B-cell involvement).
Antigen Examples:
- Poison ivy
- Jellyfish stings
- Nickel
Pathogenesis:
- A hapten penetrates the skin barrier.
- T cells migrate to the site of exposure.
- Destruction of tissue ensues at the localized site.
Diseases and Conditions:
- Poison ivy (contact dermatitis)
- Tuberculin sensitivity
Clinical Manifestations:
- Clinical signs depend on the tissue location where exposure occurred, characterized by contact dermatitis symptoms.