Hypersensitivity reactions - Type III: Nursing

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A hypersensitivity reaction is an overreaction to a foreign antigen which then causes serious damage to the body’s tissues. There are four types of hypersensitivity reactions: type I is immunoglobulin E, or IgE mediated; type II is mediated by antibodies that activate cellular cytotoxicity, type III is mediated by immune-complexes, and type IV is a delayed T cell-mediated hypersensitivity reaction. A type III hypersensitivity reaction occurs when antibodies attach to soluble antigens and form antibody-antigen complexes that float around the blood vessel, causing tissue damage depending on where they end up.
Let’s start by discussing the physiology of the humoral immune response. The humoral immune response is the part of the immune system that provides protection against invading pathogens by utilizing antibodies designed to specifically target certain antigens. It all begins in the bone marrow, where undifferentiated hematopoietic stem cells differentiate into various types of white blood cells, including and T lymphocytes, or B and T cells, for short.
When a pathogen, like a bacteria or virus enters the body, it runs into antigen-presenting cells, or APCs. APCs like macrophages or dendritic cells, then engulf and digest the pathogen, and the fragments are then presented on the APC’s surface via proteins called major histocompatibility complex class II, or MHC II. Now these fragments serve as antigens that can trigger an immune response. So the APCs present these antigens to T helper cells which have T-cell receptors, or TCRs, that recognize the antigen.