Hypersensitivity Reactions

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What is Hypersensitivity?

Hypersensitivity is the body's exaggerated immunological response to foreign bodies. Hypersensitivity is damaging to the host and is part of the protective immunity. Hypersensitivity, otherwise commonly known as "allergy", can be most easily understood, an antigenic response of the body beyond what is considered normal. It usually leads to tissue damage rather than immunity. The first exposure to the "allergen", or the antigenic substance, serves as the sensitizing dose. Any subsequent exposure to this allergen leads to hypersensitivity reaction.

There are four types of hypersensitivities:

• Type 1: Anaphylactic rx.

• Type 2: Cytotolytic, cytotoxic rx.

• Type 3: Immune complex rx.

• Type 4: Cell mediated immunity, delayed type hypersensitivity.

Hypersensitivity Reaction Format:

• The format is as follows: ANTIGEN > Immune Response > Primary Response > SP.HYPERSENSITIVITY Reaction.

• There are three types of hypersensitivity reactions: immediate (within a few minutes), intermediate (minutes to hours), and delayed (after many hours).

Types of Hypersensitivity Reactions:

• Immediate Type I IgE.

• Cytotoxic Type II Antibody and complement.

• Immune complex Type III Antigen-antobody complexes.

• Delayed Type IV T cell mediated/granulomatous.

Type 1 Hypersensitivity

Or immediate hypersensitivity, manifests itself within minutes of contact with an allergen, most commonly the second contact, the first contact being the sensitization step. Antibody IgE is produced together with release of vasoactive amines and other mediators from mast cells, and recruitment of other inflammatory cells. There are two phases: initial phase and late phase. Initial phase is characterized by vasodilatation, vascular leakage, smooth muscle spasm and glandular secretions. Late phase is characterized by infiltration of tissues with eosinophils, neutrophils, basophils, monocytes, and CD4+ T cells with tissue destruction.

Findings in Type 1 Hypersensitivity:

Vascular dilation, edema, smooth muscle contraction, mucous production, and inflammation.

Additional Type 1 Reaction Facts:

• Commonly called an allergy.

• Caused by various agents like pollen and insect bites.


• Mediated by IgE Ab that binds to mast cells and basophils through Fc portion.

• Phases are: sensitization, activation, and effector.


• There is constriction of bronchioles and bronchii due to construction of smooth muscle and dilatation of capillaries.

• There is release of epinephrine.

Schultz-Dale phenomenon:

• Histamine mimics systemic changes of anaphylaxis.

• Caused when non-exposed animal is exposed to antigens.

Type II Hypersensitivity

Involves complement fixation following antibody attachment to cell surface antigens with resultant lysis of the antibody-coated cell, is referred to as cytotoxic reaction. Blood group incompatibility due to groups and Rh factor, and drug reactions are few examples. Type II reaction is seen in autoimmune hemolytic anemia and Goodpasture syndrome. Production of IgG and IgM binds to antigen or target cell or tissue. The cell is later phagotysed.

Findings in Type II Hypersensitivity:

Cell lysis and inflammation.

Type III Hypersensitivity

Or immune complex reaction involves deposition of antigen-antibody complexes in various tissues, complement activation and triggering inflammatory response against the site of deposition. This is seen in systemic lupus erythematosus.

Findings in Type III Hypersensitivity:

Necrotizing vasculitis and inflammation.

Type IV Hypersensitivity

Or delayed hypersensitivity reaction is T cell mediated. Cytokines released by the lymphocytes are the major factors in this type of reaction. Diseases include:

Type IV Hypersensitivity Diseases:

Contact dermatitis, Mantoux skin test for Tuberculosis, type I diabetes, Multiple Sclerosis, transplant rejection, and TB.

Findings in Type IV Hypersensitivity:

Perivascular cellular infiltrates; edema; cell destruction; granular inflammation.

Additional Readings:

Basic Immunology

1. Introduction to Immunology
2. Cells of Immunology
3. Selection of Lymphocytes
4. Primary Response to Antigen
5. Antigen Processing and Presentation
6. Humoral Effector Mechanism Generator
7. Cell-Mediated Effector Mechanism Generator
8. Vaccination and Immunotherapy
9. Immunodeficiency Diseases
10. Acquired Immunodeficiency Syndrome
11. Hypersensitivities and Autoimmunity Diseases
12. Immunology of Transplantation
13. Immunology of Cancer
14. Immunology Laboratory Technology
15. Acquired Immunity
16. Type II Hypersensitivity Reaction
17. Hypersensitivity Reactions
18. Primary Immunodeficiency
19. Secondary Immunodeficiency
20. Type III Hypersensitivity Reaction
21. Type IV Hypersensitivity Reaction
22. Type V Hypersensitivity Reaction
23. Tumor Immunology
24. Images of Antibodies
25. Th1 vs Th2 cells

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1. Histology of Lymphoid Tissue

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