What is hypersensitivity? • the violent reaction of the immune system leading to severe symptoms and even death in sensitised animal when it is re-exposed to the same antigen for the second time. It is nothing but allergy. • Extreme sensitivity to antigens. • Immune response is always directed towards the protection of the host. • But in hypersensitivity the immune response becomes injurious to the host. • Hence, the immune response becomes a destructive process in hypersensitivity.
In protective immune response, the antigen or bacteria or virus is killed or neutralized. • But in hypersensitivity, the cells of the host are killed of the host itself is damaged or killed. • Hypersensitivity is the changed reactivity of the immune system. It is a beneficial protective system gone out of order. • Situation in which host immune response contribute to tissue injury are collectively referred to as hypersensitivity states.
The factors causing hypersensitivity are called allergens. • In clinical terms, hypersensitivity is called allergy.
Factors causing hypersensitivity • Drugs • Air borne particles • Infectious organisms • Food
Types of hypersensitivity • Type I: Immediate hypersensitivity reactions. • Type II: Antibody Dependent Cytotoxic Hypersensitivity Reactions • Type III: Immune-Complex mediated Hypersensitivity Reactions. • Type IV: Cell- Mediated Hypersensitivity Reactions
Type I: Immediate hypersensitivity reactions • is mediated by IgE antibodies which have bound to the Fc receptors of circulating basophils and tissue mast cells, thereby sensitizing them. • When antigen contacts the sensitized cells, an immediate skin reaction occur leading to intense local inflammation typified by diffuse infiltrations by polymorphonuclear leukocytes, • which results in a soft, swollen red skin reaction.
The chemical basis of reaction is the release of histamine, serotonin, and heparin by the sensitized basophils and mast cells. • Besides skin reaction it also affects smooth muscles, vessel walls characterized by the symptoms of irritation, rashes, swelling, wheezing shock and occasionally death - anaphylactic reactions • Eg. Allergic reactions to mosquitoes and wasps bites
Type II: Antibody Cytotoxic Hypersensitivity Reactions • is mediated by IgM and IgG antibody reacting with cellular or particulate antigens. • This antibody-dependent cytotoxic hypersensitivity results in complement-mediated cytolysis, • antibody-dependent cell mediated cytotoxicity (ADCC), or the opsonization and increased phagocytosis of particles sensitized by antibody or fixed complement. • Type II hypersensitivity can also damage the host because of circulating antibodies.
Type II hypersensitivity reactions differ from type I in three different ways: • Antibody combine with antigenic determinants on the cell surface through its Fab portion before any Fc interaction occurs. • Interaction of antibody with the target cell directly result in cell death. • IgE is not involved
Type III: Immune-Complex mediated Hypersensitivity Reactions • mediated by IgG and IgM antibodies which react with soluble antigens. • This causes the formation of circulating immune complexes (IC) which can cause widespread inflammatory responses called serum sickness • Bacterial toxins and free viral proteins are presumably the intended targets, • can cause severe organ dysfunction such as kidney failure, rheumatoid arthritis, and types of toxic shock syndrome.
Type IV: Cell- Mediated Hypersensitivity Reactions • mediated by sensitized T-lymphocytes which can cause direct target cell-mediated lysis and the release of soluble lymphokines. • T cell arrive at a skin containing antigen after infection producing a typical hard swelling -called delayed reaction. • cytotoxic T-lymphocytes are important in killing cells infected with intracellular parasites such as viruses, bacteria, and trypanosomes as well as fungi and perhaps cancer cells.
T cell mediated hypersensitivity is responsible for accelerated graft reaction and allergic reactions caused by contact with chemical or metal allergens called- contact sensitivity.