Hypersensitivity refers to an inappropriate or undesirable immune response ( damaging immunological reactions to extrinsic antigens)

Hypersensitivity reactions require a pre-sensitized (immune) state of the host, genetically susceptible individual called atopic individuals.

Immediate hypersensitivity or allergy due to overproduction of IgE antibody on mast cells or basophiles

Rapid reaction : the reaction usually takes 5 - 10 minutes from the time of exposure to the antigen, although sometimes it may have a delayed onset (10 - 12 hours).

The initial responseThe initial response, , characterized by vasodilatation, vascular characterized by vasodilatation, vascular leakage, & smooth muscle spasm, leakage, & smooth muscle spasm,

Immediate hypersensitivity is mediated by IgE.

The primary cellular component in this hypersensitivity is the mast cell or basophil.

Prick test is used to detect type I “ wheal and flare”

Allergy disease are common about 15-20%of the population

Diagnostic tests for immediate hypersensitivity include skin (prick and intradermal) tests

Anaphylaxis case Bee stingAllergic asthma

Allergic dermatitis

Asthma is defined as increased irritability of the bronchial tree with paroxysmal narrowing of the airways.which may reverse spontanously or after treatment. Rarely, symptoms persist for which may reverse spontanously or after treatment. Rarely, symptoms persist for days, leading to respiratory failure and even death.days, leading to respiratory failure and even death.

Features

-Increased irritability of bronchi causes bronchospasm

-Paroxysmal attaches

-Over distended lungs

-Mucus plugs in bronchi

-Enlarged bronchial mucous glands

Classification

There are five major clinical categories of asthma

► atopic

► non-atopic

► aspirin-induced

► occupational

► allergic bronchopulmonary aspergillosis.

Acute attacks are characterized byAcute attacks are characterized by

- -a feeling of tightness in the chesta feeling of tightness in the chest - - difficulty in breathingdifficulty in breathing

- - particularly in exhaling which accompanied by loud particularly in exhaling which accompanied by loud wheezing and coughing wheezing and coughing

Pathogenesis of allergic asthma. Inhalation of allergen causes degranulation of mast cells bearing specific IgE molecules. Release of vasoactive substances from the cells causes bronchial constriction, oedema and mucus hypersecretion.

SRS-A = slow –reacting substance of anaphlylaxis

ECF-A = eosinophil chemotactic factor of anaphylaxis

PAF = platelet activation factor

Tracheal mucus plug in death from status asthmaticus.

This cast of the bronchial tree is formed of inspissated mucus and was coughed up by a patient during an asthmatic attack. The outpouring of mucus from hypertrophied bronchial submucosal glands, the bronchoconstriction, and dehydration all contribute to the formation of mucus plugs that can block airways in asthmatic patients

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Histological section of lung at autoopsy showing occlusion of airways by oedma and mucus plugs accompanied by alveolar distension with entrapped gas.

The lumen (top) of this small bronchus containing exudate in which there are many cells mostly leukocytes. The epithelial lining consist of large mucus secreting cells. The epithelial basement membrane is thickened and the smooth muscle of the wall is hypertrophied ,eosinophil leukocytes and lymphocytes are also present in the wall.