Hypersensitivity

Terminology description

When an individual is not immunopotent to control the secondary response, they may develop hypersensitive reactions. Antigen amount is more which can result in stimulation of effector mechanism effectively.  This sort of immune response may damage the tissues. It may be due to direct interaction between provocative elements and innate immune system. The research in hypersensitivity was first persuaded by Richet and Portier.

Types of hypersensitivity

Following are types of hypersensitivity.

• Anaphylactic hypersensitivity (Type I)
• Antibody dependent cytotoxic hypersensitivity (Type II)
• Complex mediated hypersensitivity (Type III)
• Cell mediated hypersensitivity or delayed type hypersensitivity (Type IV)
• Stimulatory hypersensitivity (Type V)

Type I hypersensitivity or Anaphylactic hypersensitivity: Anaphylaxis contracts smooth muscles. Capillaries are dilated depending on the interaction of specific antigen with antibody (IgE). This interaction gives receptor of high affinity. Receptors of IgE by clustering or cross linking, activate tyrosine protein, kinase. This results in mediator’s release of leukotrines, PAF (platelet activating factors), histamine, chemotactic factors like neutrophil and eosinophil, cytokine interleukin factors like 3, 4 and 5 and GM CSF.

Type II hypersensitivity or Antibody dependent cytotoxic hypersensitivity: Antibody of dead cells remains associated with antigenic surface. Cells are attached to phagocytic cells by IgG. By an extracellular mechanism, IgG destroys K cells, macrophages and polymorphic cells. Instances of type II hypersensitivity are hemolytic disease, graft rejection mediated by antibody production, transfusion reaction and autoimmune reactions.

Type III hypersensitivity or Complex mediated hypersensitivity: Antigen antibody interaction results in the following effects.

• Complement activation
• Polymorphic nuclear leukocytes attraction
• Proinflammatory cytokine release by macrophage stimulation
• Microthrombi and vasoactive amine release by platelet aggregation

Instances of complex mediated hypersensitivity are pigeon fancier disease, farmer’s lung, pulmonary aspergillosis, glomerulonephritis, systemic lupus erythematosus and hemorrhage shock.

Type IV hypersensitivity or Cell mediated hypersensitivity: Interaction of T cells with antigen may endanger tissues. As a result of cell mediated hypersensitivity, cytokines are released. Type IV diseases are

• Bacterial diseases: leprosy, tuberculosis
• Fungal diseases: histoplasmosis, candidiasis
• Parasitic diseases: schistosomiasis, leishmaniasis
• Viral diseases: herpes, measles
• Insect bites
• Poison ivy
• Contact dermatitis
• Inflammatory bowel disease

Type V hypersensitivity or Stimulatory hypersensitivity: When hormone receptor interacts with antibody, it stimulates more number of cells. Grave’s diseases are stimulated hypersensitivity type. Thyroid stimulating hormone binds to its own receptor and activates adenyl cyclase. Second messenger cAMP stimulates the thyroid cells.