1. Hypersensivity Reactions
Allergies Greek = altered reactivity
1906 – von Pirquet coined term: hypersensitivity
Hypersensitivity reactions – ‘over reaction’ of the immune system to harmless environmental antigens

2. Hypersensitivity reactions – originally divided into 2 categories: immediate and delayed In 1968 Coombs & Gell defined the 4 types used today
Type I: classical immediate hypersensitivity
Type II: cytotoxic hypersensitivity
Type III: immune-complex mediated hypersensitivity
Type IV:cell mediated or delayed hypersensitivity

3. Hypersensitivity reaction depends on:
1) chemical nature of allergen
2) route involved in sensitization ie inhalation, ingestion, injection…
3) physiological state of individual / genetic potential

4. Type I Common among population in developed nations
Prerequisite: need prior sensitization to antigen
the binding of antigen to antigen specific IgE bound on mast cells
Rapid liberation of active chemicals such as histamine and serotonin

5. Typical responses to these chemicals: Increased capillary permeability
Urticaria [hives]
Excessive mucus production
Allergic rhinitis [hay fever]
Diarrhea or vomiting
Asthma

6. Type II
small molecules bound to cells and make a structure perceived as foreign by immune cells
[ blood transfusion reactions. Erythroblastosis fetalis]
Allergens create a situation that induces cytolysis or cytotoxicity.
Antibodies involved are IgG & IgM
Complement is also involved

7. Complement
Blood proteins – initiate a series of enzymatic reactions leading to the ‘fixing’ of complement fragments to the pathogen’s surface – tagging it for destruction
Allergens trigger the classical complement pathway: antibody binds to specific antigen  recruitment of inflammatory cells, opsonization  facilitating phagocytosis

8. Type III
soluble protein complexes found in blood bound to IgG [ when non human proteins are given therapeutically – can be side effect]
Cause acute inflammatory reactions
Immune complexes can become deposited in walls of small blood vessels in alveoli  anaphylaxis

9. Type IV caused by products of antigen-specific effector T cells
T cells undergo blastogenesis and cellular division  production of reactive cells
Usually takes 24 – 28 hours
No histamine or chemically related substances are released from cells