Allergy and Hypersensitivity

Allergy and Hypersensitivity

• Allergies are conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment.
• Common allergens include pollen and certain foods.

Hypersensitivity

• Hypersensitivity is a reflection of excessive or aberrant immune responses.
• Hypersensitivity reactions may occur in two situations:
  • Responses to foreign antigens may cause tissue injury, especially if the reactions are repetitive or poorly controlled.
  • The immune responses may be directed against self-antigens, as a result of the failure of self-tolerance.

Types of Hypersensitivity Reactions

• Type I: Immediate Hypersensitivity
  • Caused by the release of mediators from mast cells.
  • Depends on the production of immunoglobulin E (IgE) antibodies.
• Type II: Antibody-Mediated
  • Antibodies directed against cell or tissue antigens can damage these cells or tissues or impair their function.
• Type III: Immune Complex Diseases
  • Antibodies against soluble antigens in the blood may form complexes with the antigens, causing inflammation and tissue injury.
• Type IV: T cell-mediated diseases
  • Some diseases result from the reactions of T lymphocytes specific for self-antigens or microbes in tissues.

Immediate Hypersensitivity

• Immediate hypersensitivity is an IgE antibody- and mast cell-mediated reaction to certain antigens.
• Causes rapid vascular leakage and mucosal secretions, often followed by inflammation.
• Allergy/Atopy: Disorders in which IgE-mediated immediate hypersensitivity is prominent.
• Atopic: Individuals with a propensity to develop these reactions.

Clinical Syndromes and Therapy

• Common types of immediate hypersensitivity:
  • Hay fever
  • Food Allergies
  • Asthma
  • Anaphylaxis
• Allergies are the most frequent disorders of the immune system, estimated to affect 10% to 20% of people.
• The incidence of allergic diseases has been increasing, especially in industrialized societies.

Development of Immediate Hypersensitivity Reactions

• Sequence of events includes:
  • Activation of Th2 and IL-4-secreting follicular helper T (Tfh) cells.
  • Binding of IgE to IgE-specific Fc receptors of mast cells.
  • Cross-linking of the bound IgE by the antigen.
• During this event, mast cells mediators cause a rapid increase in vascular permeability and smooth muscle contraction.
• Immediate reactions may occur in minutes.
• Late-phase reactions happen when mast cells continuously recruit neutrophils and eosinophils over several hours, causing tissue injury.