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Tolerance And Immunity-Central Tolerance-Peripheral Tolerance-Importance And Factors Influence Tolerance Induction Tolerance * It is specific immunologic unresponsiveness that is the absence of specific immune responses to a particular antigen in a fully immunocompetent person * Unresponsiveness to self-antigens is known as auto tolerance and it is a normal physiological condition. * Both B-cells and T-cells participate in the process of tolerance * But T-cells play the primary role Central Tolerance Clonal–deletion: * It is the process by which T-cells acquire the ability to distinguish self from non-self, in fetal thymus * This involves the killing of T-cells that react against antigens present in the fetus at that time * Clonal deletion is a process by which B cells and T cells are deactivated after they have expressed receptors for self-antigens and before they develop into fully immunocompetent lymphocytes. Peripheral Tolerance T-cell tolerance (clonal anergy): * Som...

TYPE 4 (T CELL-MEDIATED) HYPERSENSITIVITY-Mechanism and Diseases It is initiated by activated T-lymphocytes (Either helper T cells or cytotoxic T cells). On the basis of this it has two forms: 1)-Delayed type hypersensitivity (mediated by helper T or CD4+ T cells) 2)-T-cell mediated cytotoxicity (mediated by CD8+ T cells or Cytotoxic T Cells) 1)- Delayed-type hypersensitivity (mediated by helper T or CD4+ T cells)

TYPE 3 (IMMUNE COMPLEX MEDIATED)HYPERSENSITIVITY-Pathogenesis-Causes-Effects-Diseases Ag = Antigen Ab= Antibody Ag-Ab complexes are formed in this disorder which deposits in various tissues and induce inflammation in them and gradually destroys them. Ag-Ab complexes formation is a normal phenomenon in most of the situations but they do not deposit normally. It is the size of the complexes or the functional or structural abnormality of the mononuclear phagocytic cells which render the Ag-Ab complexes to be deposited in various locations. Charge and three-dimensional structure of the immune complexes, the valency of the Ag, the affinity of the Ag to various tissue components and hemodynamic factors also play roles in the deposition of the complexes.

Type 2 ( Antibody Mediated ) Hypersensitivity Reaction-Mechanism-Diseases and Examples •Type 2 hypersensitivity reaction is mediated by antibodies directed against the Antigens (intrinsic or extrinsic and normal or altered) present on the cell surface or extracellular matrix. MECHANISM OF TYPE II (ANTIBODY MEDIATED) HYPERSENSITIVITY There are three mechanisms of type two hypersensitivity reactions. Each Mechanism gives rise to different clinical manifestation. •1)- Opsonization of cells by antibodies and complement components and ingestion by phagocytes. In this mechanism, antibodies are formed against antigens present on the cell surface. Antibodies opsonize the antigens for phagocytosis. Moreover, these Antibodies also stimulate the complement system which further opsonizes the cells for phagocytosis). Opsonization is a process in which antibodies combine with antigens and which are then recognized by the phagocytes for phagocytosis. •2)- Inflammation induced by antibodies binding...

Type One Hypersensitivity Reaction-Causes-Types-Phases-Mediators and Examples Of Type One Hypersensitivity Reaction Type I Hypersensitivity Reaction: •A rapidly developing immunologic reaction occurring within minutes after the combination of an antigen (allergen in this case) with antibody bound to mast cells in individuals previously sensitized to that antigen. These reactions are often called allergies. Key: Ig= Immunoglobulin ,  Abs = Antibodies,  Ags= Antigens, IgE = Immunoglobulin E, Pathogenesis Of Type One Hypersensitivity Reaction •Primary response (Sensitization of the person): –Antigens (Allergens) bind to B lymphocytes & stimulate them. They form both plasma cells and memory cells. The plasma cells produce IgEs (attach to mast cells in the vicinity) while the memory cells remain as armed guards in that area. This primary response is usually weak and subsides by itself if the dose of the Ags is small or the exposure is single and for a short time but the Me...