Type I Hypersensitivity Reactions

Hypersensitivity Reactions Overview

• Hypersensitivity reactions are exaggerated immune responses that can result in inflammation and damage to various tissues or organs, potentially manifesting as a range of clinical symptoms. These reactions occur when the immune system becomes overly sensitive to substances that are typically harmless, known as allergens.
• There are four principal types of hypersensitivity reactions: Type I (Immediate), Type II (Cytotoxic), Type III (Immune Complex), and Type IV (Delayed). Each type is characterized by distinct mechanisms and clinical outcomes, involving different components of the immune system.

Type I Hypersensitivity Reactions

• Type I hypersensitivity reactions are primarily mediated by Immunoglobulin E (IgE) antibodies that react against specific allergens. These allergic responses often occur within minutes of exposure to the allergen, hence the term "immediate." Exposure can occur through various routes, such as inhalation, ingestion, or direct contact.
• Key cytokines involved in this process include Interleukin-4 (IL-4) and Interleukin-5 (IL-5), which play critical roles in stimulating the production of IgE and promoting the maturation of B lymphocytes into IgE-secreting plasma cells. This cytokine signaling is pivotal for the development and enhancement of allergic responses.
• Once IgE is produced, it binds to high-affinity IgE receptors (FcεRI) on the surface of mast cells. Subsequent exposure to the same allergen leads to the cross-linking of IgE on mast cells, causing them to degranulate. This process releases a variety of mediators including histamine, leukotrienes, and prostaglandins, which contribute to the allergic symptoms experienced by the individual.
• Clinical manifestations of Type I hypersensitivity range from localized allergic reactions such as itching, urticaria (hives), and allergic rhinitis to more severe systemic responses such as anaphylaxis. Anaphylaxis is a life-threatening allergic reaction characterized by symptoms such as difficulty breathing, swelling of the throat, rapid drop in blood pressure, and potential loss of consciousness, necessitating immediate medical intervention.

Type II Hypersensitivity Reactions

• Type II hypersensitivity reactions are characterized by the destruction of target cells through the binding of antibodies to specific antigens located on the surface of these cells. This binding triggers various pathogenic mechanisms that ultimately lead to cell lysis or functional impairment.
• Common target antigens in Type II reactions include human leukocyte antigens (HLA), tissue-specific antigens, and some drugs and their metabolites. These antigens can be derived from autoimmune processes or can result from the introduction of foreign substances (like transfused blood) that mimic human antigens.
• Pathophysiological mechanisms for Type II hypersensitivity include:
  • Complement-mediated lysis: The classical complement pathway may be activated by antibody-antigen complexes, leading to the formation of the membrane attack complex, resulting in the lysis of the target cell.
  • Phagocytosis by macrophages: Antibody-coated cells may be recognized by phagocytes, which ingest and destroy the target cells.
  • Antibody-dependent cell-mediated cytotoxicity (ADCC): Natural killer (NK) cells can recognize and kill antibody-bound target cells, contributing to the cell's destruction.

Type III Hypersensitivity Reactions

• Type III hypersensitivity reactions are a result of immune complex formation, where antigen-antibody complexes form in circulation and subsequently deposit in various tissues. This accumulation triggers an inflammatory response that can lead to significant tissue damage.
• Common antigens involved in Type III reactions include those derived from viral, bacterial, and parasitic infections, as well as environmental factors such as pollen or autoantigens from self-tissues. The formation of these immune complexes can occur after the presence of infections or as a result of autoimmunity.
• Pathophysiology of Type III reactions typically involves:
  • Immune complexes deposit in tissues such as the kidneys and blood

This quiz focuses on the pathophysiology of Type I hypersensitivity reactions, including the roles of dendritic cells, Th2 cells, and cytokines. Understand the immune responses involved in allergen presentation and IgE production. Test your knowledge about the mechanisms underlying hypersensitivity reactions.