Drug allergy: type I, II,III
Type I hypersensitivity disorder
A Hypersensitivity disorder is where the immune system reacts in such a way to damage the organism rather than successfully protecting it. These disorders are classified into four different groups, Types I-IV, which are dealt with the this and the following three questions. Here we will consider Type I Hypersensitivity.
Type I hypersensitivity is also known as Immediate hypersensitivity, as the reaction is instantaneous. It is the typical ‘Allergic’ response which one might think of, and is responsive to Antihistamines, while the other three types are not! This type of sensitivity sees the generic redness, swelling, inflammation and potentially anaphylactic shock that we associated with an allergic reaction, but it does not involve the complement system.
As with all of the hypersensitivity reactions we will address in this section, hypersensitivity is a two step process, with first a First Exposure (Sensitization) phase, where the body is primed to react to the antigen, and secondly a Subsequent exposure phase, where the actual allergic reaction occurs. We will explore this in more detail in the Pathogenesis of the disorder.
Antigens (Allergens)
- Insect venom
- Food
- Pollen
- Dust mites
Antibody
- Type I is a humoral immune response, mediated by Immunoglobulin E (IgE)
Pathogenesis
Type I hypersensitivity is a result of a genetic defect that makes T-cells over sensitive to a certain allergen. Based on the above ‘First Exposure’ and ‘Subsequent Exposure’ model above, here we will consider the mechanism of hypersensitivity
- First Exposure
- Antigen enters body and is picked up by Macrophages/Dendritic cells
- These carry antigen to lymph nodes which where TH-cells are overstimulated
- TH-cells become activated, which in turn activate B-cells, turning them into plasma cells
- Active B-cells (now known as plasma cells) now produce antigen specific IgEs, which bind to Mast Cells ‘priming’ it
- Mast cells now remain in the body, ‘geared up’ to attack the antigen
- Subsequent exposure
- Antigen enters the body once more, Mast Cells with IgE antibodies now bind to antigen
- This binding causes the degranulation of Mast Cells, releasing lots of Histamine and other inflammatory mediators
- Results in inflammation, bronchoconstriction, edema and urticaria
Treatment
- Adrenaline followed by corticosteroids
Diseases
- Atopic asthma
- Anaphylactic shock
- Hay fever
- Atopic dermatitis
Type 2 Hypersensitivity disorder
Type II hypersensitivity reactions are also known as Cytotoxic reactions. This type of hypersensitivity effectively sees the destruction of cells (and thereby tissues), mediated by antibodies.
Antigen
- Often drugs, such as penicillin