Hypersensitivity Reactions, Allergy, and Autoimmune Diseases in Microbiology 19

Hypersensitivity Reactions CH 19

Overview of Hypersensitivity

Hypersensitivity refers to exaggerated or inappropriate immune responses to antigens (allergens), which can lead to tissue damage. These reactions are classified based on their mechanisms and timing.

• Types of reactions:

  • Anaphylactic

  • Cytotoxic

  • Immune complex

  • Delayed cell-mediated

Types of Hypersensitivity

The following table summarizes the types of hypersensitivity reactions and the time after exposure for clinical symptoms to appear:

Type of Reaction	Time After Exposure for Clinical Symptoms
Type I (anaphylactic)	<30 min
Type II (cytotoxic)	5-12 hours
Type III (immune complex)	3-8 hours
Type IV (delayed cell-mediated)	>1 day

Type I (Immediate) Hypersensitivity

Definition and Mechanism

Type I hypersensitivity involves localized or systemic reactions resulting from the release of inflammatory molecules in response to an allergen. These reactions develop within seconds or minutes following exposure to an allergen.

• Allergens: Commonly called allergens, these are antigens that stimulate an allergic response.

Type I Anaphylactic Reactions

• Cells involved: Mast cells and basophils

• Mechanism: IgE antibodies bind to mast cells and basophils, causing degranulation and release of mediators.

• Key mediators:

  • Histamine

  • Prostaglandins

  • Leukotrienes

  • Serotonin

  • Bradykinin

Clinical Signs of Localized Allergic Reactions

• Usually mild and localized

• Site of reaction depends on the portal of entry

• Upper respiratory tract response: watery nasal discharge, sneezing, itchy throat and eyes, excessive tear production

Clinical Signs of Systemic Allergic Reactions

• Degeneration of many mast cells at once causes the release of large amounts of histamine and inflammatory mediators

• Anaphylactic or anaphylactic shock can result

• Constriction of smooth muscle contracts violently

• Leakage of fluid from blood vessels causes swelling of the larynx and other tissues

• Bronchial smooth muscle constriction

• Must be treated promptly with epinephrine

Allergy "Vaccines"

• Desensitization or hyposensitization

• Therapeutic method to prevent reactions between allergen, IgE, and mast cells

• Involves administration of increasing doses of allergen to stimulate formation of IgG antibodies

• IgG antibodies intercept the allergen, preventing it from binding to IgE

Type II Hypersensitivity: Cytotoxic Reactions

Definition and Mechanism

Type II hypersensitivity involves complement-assisted destruction (lysis) of cells by antibodies (IgG and IgM) directed against those cells' surface antigens.

• Examples:

  • Transfusion reactions

  • Some syndromes of autoimmune reactions

• Alloantigens: Molecules that differ in the same species and are recognized by lymphocytes of the recipient

Type III Hypersensitivity: Immune Complex Reactions

Definition and Mechanism

Type III reactions occur when antigen-antibody complexes form and deposit in tissues, leading to inflammation and tissue damage.

• Commonly seen in autoimmune diseases

• Antigen-antibody complexes lodge in basement membranes

Type IV Hypersensitivity: Delayed Cell-Mediated Reactions

Definition and Mechanism

Type IV reactions are mediated by T cells responding to antigens displayed on self tissues or transplanted foreign cells. These reactions are delayed, typically appearing more than 24 hours after exposure.

• Examples: Contact dermatitis, graft rejection

Autoimmune Diseases

Definition

Autoimmune diseases occur when the body produces antibodies and cytotoxic T cells that attack normal body cells. These diseases often develop spontaneously and randomly.

• More common in older individuals

• Some features are common among different autoimmune diseases

• More common in women than men

Examples of Autoimmune Diseases

• Systemic Lupus Erythematosus (SLE):

  • Characteristic butterfly-shaped rash across the nose and cheeks

  • Autoantibodies against organs, tissues, or intracellular materials

• Rheumatoid arthritis:

  • Autoantibodies against connective tissue

Graft Rejection

Definition and Mechanism

Graft rejection is an immune response against foreign tissue or organs that have been transplanted into the body. It is a normal immune response against foreign major histocompatibility complex (MHC) antigens.

• Likelihood of rejection depends on the degree to which the graft is foreign to the recipient

• Graft rejection is attributed to cytotoxic T-cell action

Types of Grafts

Type	Description
Autograft	Use of one's own tissue
Isograft	Use of tissue from another person (genetically identical)
Allograft	Use of tissue from another person (not genetically identical)
Xenotransplantation	Use of tissue from a nonhuman source
Privileged sites	Sites where grafts are not likely to be rejected

Privileged Sites

• Sites such as the cornea and certain blood vessels are less likely to be rejected due to low levels of immune response

• Some sites lack lymphatic drainage and have high levels of immunosuppressive molecules

Immunosuppressive Processes

Prevention of Graft Rejection

• Prevents an immune response to transplanted tissues

• Immunosuppressive drugs inhibit T cell and B cell reproduction

• Examples: Cyclosporine, Sirolimus, Blockers of IL-2

Summary Table: Hypersensitivity Types

Type	Mechanism	Examples
I (Anaphylactic)	IgE-mediated, mast cell degranulation	Allergic rhinitis, asthma, anaphylaxis
II (Cytotoxic)	IgG/IgM-mediated, complement activation	Transfusion reactions, hemolytic disease of the newborn
III (Immune Complex)	Antigen-antibody complex deposition	Serum sickness, Arthus reaction
IV (Delayed)	T cell-mediated	Contact dermatitis, graft rejection

Additional info: The notes have been expanded to include definitions, examples, and mechanisms for each type of hypersensitivity and autoimmune disease, as well as tables for clarity and comparison.