Hypersensitivity

Hypersensitivity : An exaggerated or inappropriate immune response that results in reactions harmful to the host.
Hypersensitivity reactions can be subdivided into four main types. Types I, II, and III are antibody-mediated, whereas type IV is cell-mediated.

Type I: Immediate (Anaphylactic) Hypersensitivity
The process begins when an antigen (Allergen) induces the formation of IgE antibody which binds firmly by its Fc portion to receptors on the surface of basophils and mast cells.
Re-exposure to the same antigen results in cross-linking of the cell-bound IgE, degranulation, and release of pharmacologically active mediators within minutes (Immediate phase).

Immediate(Anaphylactic) hypersensitivity
Symptoms such as edema and erythema ("wheal and flare") and itching appear rapidly because these mediators are pre-formed in mast cell, e.g., Histamine, and Serotonin .
Histamine release causes vasodilation, increased capillary permeability, and smooth-muscle contraction.
The (late phase) of IgE-mediated inflammation occurs approximately 6 hours after exposure to the antigen and is due to mediators, e.g., leukotrienes (SRS-A) & prostaglandin that are synthesized after the cell degranulates. These mediators cause an influx of inflammatory cells, such as Neutrophils and Eosinophils, and symptoms such as erythema and induration occur.
Prostaglandins and leukotrienes cause dilation and increased permeability of capillaries and bronchoconstriction.
Complement is not involved with either the immediate or late reactions because IgE does not activate complement, but complement factors C3a and C5a (anaphylatoxin) can cause vasodilation and increasing capillary permeability.
The allergens involved in hypersensitivity reactions are substances, such as pollens, animal danders , foods (nuts, shellfish), and various drugs, to which most people do not exhibit clinical symptoms. However, allergic individuals respond to those substances by producing large amounts of IgE and, as a result, manifest various allergic symptoms. Non-allergic individuals respond to the same antigen by producing IgG, which does not cause the release of mediators from mast cells and basophils because there are no receptors for IgG on those cells.
Clinical manifestation:
Atopy is the term can include the clinical manifestations of type I hypersensitivity which can appear in various forms, e.g., Urticaria (also known as hives), eczema, angioedema, rhinitis and conjunctivitis (also known as hay fever), and asthma. Which clinical manifestation occurs depends in large part on the route of entry of the allergen and on the location of the mast cells bearing the IgE specific for the allergen.
The most severe form of type I hypersensitivity is Systemic anaphylaxis, in which severe bronchoconstriction and hypotension (shock) can be life-threatening. The most common causes of anaphylaxis are foods, such as peanuts and shellfish, bee venom, and drugs.
Anaphylactoid : …………???????????????