Lecture 18: Immune Disorders

Hypersensitivities

  • Hypersensitivities: immune system overreacts to a something harmless

    • example: allergies

Type I Hypersensitivity

  • Type I Hypersensitivity: Triggered by allergens and mediated by Immunoglobulin E (IgE).

    • allergens: harmless antigen that rigger an inflammatory response

      • allergens Types:
          - Localized reactions (e.g. hay fever)

        • Common examples:
            - Pollen allergy

        • Systemic reactions (e.g. anaphylaxis): effects on a systemic level, inflammation throughout the body. blood vessels get leaky as once. blood pressure drops at once?


  •   - Allergic dermatitis
      - Peanut allergy

  • Allergen: A harmless antigen that provokes an inflammatory response.

Mechanism of Type I Hypersensitivity

  1. Sensitization Phase:
       - Antigen presenting cells (APCs) activate helper T cells.
       - Helper T cells activate B cells.
       - B cells transform into plasma cells that produce IgE antibodies.
       - IgE binds to mast cells (which sensitizes them to future exposures to the allergen)

  2. Subsequent Exposure:
       - When a sensitized individual encounters the same allergen, mast cells release histamine and other inflammatory molecules, leading to allergy symptoms

steps of Type 1 Hypersensitivity:

Concept Check on Type I Hypersensitivity

  • Question: What causes the release of histamine during allergies?
       - Answer:
         - Histamine is released when the allergen binds to IgE on mast cells.
         - Antibody binding triggers histamine release; it may also involve natural killer cells and complement activation, but primarily mast cells are responsible.

Type II Hypersensitivity

  • Type II Hypersensitivity: Involves the immune system attacking foreign but harmless cells     

    • antibody mediated - invlolves IgG or IgM

      • IgG or IgM antibodies bind to antigens on foreign cells, promoting their lysis and inflammation.

    • example: such as in blood transfusions).

  • Mechanisms:
      - Cytotoxic response to antigens present on cells (often in blood).

Blood Donation Example

  • Different blood types present different antigens on red blood cells

    • If a mismatched blood type is transfused, the recipient’s immune system produces antibodies against the foreign antigens.

Hemolytic Disease of the Newborn

  • Occurs when maternal antibodies target fetal red blood cells,

  • leading to severe complications,

  • necessitating close monitoring and potential treatment (e.g., blood transfusions).

Type III Hypersensitivity

  • Type III Hypersensitivity: Characterized by the formation of immune complexes (more antibodies than antigens).

    • antibody mediated - IgG antibody

    • Consequences: Immune complexes deposit in tissues, preventing phagocytosis, activating complements, and causing inflammation.

    • They may obstruct small blood vessels, leading to tissue damage.

    • Example:

      • Glomerulonephritis: inflammation of the kidney's glomerulus, often resulting from immune complexes.

Type IV Hypersensitivity

  • Type IV Hypersensitivity: A delayed immune response mediated by T cells rather than antibodies.

  • Involves inappropriate attacks on self-cells, leading to tissue damage.

Mechanism of Type IV Hypersensitivity

  1. Sensitization:
       - Antigen presenting cells (APCs) recognize and present harmless hapten antigens, activating T cells.
       - Memory helper T cells and memory cytotoxic T cells are stored.

  2. Subsequent Exposure:
       - Helper T cells stimulate an inflammatory response when they come in contact with the same hapten again.
       - Cytotoxic T cells attack body cells presenting the hapten, causing damage.

Summary of Hypersensitivities

  • Type I: Allergies

  • Type II: Cytotoxic (blood transfusions)

  • Type III: Immune complexes

  • Type IV: Delayed (T cell-mediated)

Concept Check on Hypersensitivity Types

  1. Cytotoxic T cells attacking body cells displaying harmless antigens – Type IV

  2. IgE antibodies triggering histamine release – Type I

  3. IgG antibodies attacking blood cells after transfusion – Type II

  4. Immune complexes leading to inflammatory damage – Type III

Hypersensitivity Treatments

  • Desensitization therapy: Repeated injections of diluted allergens to promote IgG production, which can bind allergens before IgE can respond.

  • Block immune response treatments:
      - Antihistamines (block histamine effects)
      - Corticosteroids (reduce inflammation)
      - Epinephrine (counteracts anaphylaxis)

Autoimmune Disorders

  • Autoimmune Disorders: Conditions where the body’s immune response mistakenly attacks its own cells.

Immunodeficiency Disorders

  • Immunodeficiency Disorders: Conditions where immune defense mechanisms are absent or dysfunctional. These can be inherited or acquired.

Concept Check on Autoimmune Disorders

  • Question: Which scenario describes an autoimmune disease?
      - The immune system attacks its own connective tissue cells (This is correct).