Understanding Hypersensitivity

• Hypersensitivity involves abnormal immune system reactions against antigens
• Reactions include allergies, which are exaggerated responses to harmless antigens
• Reactions include autoimmune diseases, which are responses against the body's own antigens
• Reactions can range from mild rashes to organ damage, and even fatal anaphylactic shock

Factors Contributing to Hypersensitivity

• An imbalance between effectors and regulators compromises the immune system, leading to overreactions
• Self-reactivity happens when T-cells and B-cells that the body should eliminate escape and attack the body's own tissues
• Immune cells typically learn not to react to the body’s own antigens during development in the thymus and bone marrow

Development of Hypersensitivity

• Reactions occur only in pre-sensitized individuals
• A previous asymptomatic contact with the antigen is required
• During this initial contact, the body produces antibodies or activates immune cells that may cause symptoms upon subsequent exposures

Types of Hypersensitivity

• Classified into four types based on mechanisms of action: Type I, Type II, Type III, and Type IV

Type I Hypersensitivity

• Previous antigen exposure leads to production of IgE antibodies
• IgE molecules bind to receptors on mast cells and basophils
• Re-exposure to the antigen makes it bind to adjacent IgE molecules, triggering histamine and inflammatory chemical release
• Released chemicals cause blood vessel dilation, smooth muscle spasms, edema, rash, breathing difficulty, abdominal cramping, vomiting, and diarrhea
• Reactions are rapid, typically occurring within minutes of exposure
• Severe reactions can result in anaphylactic shock, characterized by low blood pressure and airway constriction
• Most allergies are Type I hypersensitivity reactions

Type II Hypersensitivity

• IgG or IgM antibodies bind to antigens on the surface of cells
• Antibody binding marks cells for destruction via the complement system or phagocytosis
• Antibodies may also disrupt normal cell function without killing the cells
• Underlying mechanism for many autoimmune diseases, where the body produces antibodies to destroy its own cells
• Hemolytic disease of the newborn is an example, where maternal antibodies destroy fetal red blood cells

Type III Hypersensitivity

• IgM or IgG antibodies bind to free-floating antigens, forming antibody-antigen complexes
• Complement system is activated, causing inflammation and tissue damage
• Serum sickness is an example, caused by a large amount of antigens in the blood
• Immune complexes deposit in blood vessel walls, causing vasculitis

Type IV Hypersensitivity

• A delayed reaction mediated by T-cells
• Pre-sensitized T-cells are produced during a previous exposure to the antigen
• Re-exposure to the antigen causes T-helper cells to release inflammatory cytokines, while T-killer cells induce cytotoxic reactions
• Contact dermatitis, caused by direct skin contact with substances, is a typical example
• Basis of the tuberculosis skin test