Ans. is 'b' i.e., Mast cells Type I hypersensitivity (Immediate hypersensitivity) o It is a rapidly developing immunological reaction occuring within minutes after the combination of an antigen with antibody bound to mast cells in individuals previously sensitized to the antigen. o Immediate hypersensitivity reactions are mediated by Ig E, but T and B cells play impoant role in the development of these antibodies. o Type I reaction requires prior sensitization to a specific antigen (allergen). o Most exposures occur either by inhalation (respiratory route) or ingestion (GIT) of antigen. o Type one hypersensitivity has two phases :- 1.Initial response o After first antigen exposure, this antigen is presented to CD-4 helper T cells (TH2 type) by antigen presenting cells. These primed TH2 cells release IL-4 that acts on B-cells to form Ig E specific for that paicular antigen. The antigen specific lg E antibodies then bind to the surface receptors of mast cells and basophils. The process from first antigen exposure to the coating of mast cells by Ig E acts as sensitization (prior sensitization) and first exposure is also called priming or sensitizing exposure (dose). Subsequent exposure (shocking dose) to same antigen then result in activation of mast cells and basophils with release of mediator from these cells that leads to ? Smooth muscle spasm Increase mucus secretion from the epithelial cells. Vasodilatation Increased vascular permeability Recuritment of inflammatory cells Note - Most of the clinical effects are due to smooth muscle spasm. 2. Late phase response In this phase additional leukocytes are recruited. These cells amplify and sustain the inflammatory response without additional exposure to the triggred antigen. Among the cells that are recruited in the late-phase reaction, eosinophils are paicularly impoant. Eosinophils produce major basic protein and eosinophilic cataionic protein that are toxic to epithelial cells. Platelet activating factor (PAF) recruits and activates inflammatory cells and is most impoant cytokine in the initatiation of late phase response. Type I hypersensitivity occurs in two forms Anaphylaxis --> Acute, potentially fatal, systemic. Atom --> Chronic, Nonfatal, Lacalized. Remember o Mast cells are the most impoant cells in type I hypersensitivity. o Eosinophils are the most impoant cells in the late phase reaction of type I hypersensitivity. o Histamine is the most impoant mediator in type I hypersensitivity. o IL-4 is paicularly impoant, it is essential for turning on IgE secreting B cells. o PAF is the most impoant cytokine in the initiation of late phase reaction. o Most potent eosinophilic activating cytokine is IL-5.