Hypersensitivity reactions - Type II: Nursing

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 II, or cytotoxic, hypersensitivity reaction occurs when IgG or IgM antibodies bind to an antigen on the surface of a cell and cause other immune cells and complements to attack it, leading to the cell’s destruction.

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 T and B lymphocytes, or T and B 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 which are any thing that could 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. These T helper cells will go on to activate specific B cells which causes them to transform into plasma cells that could produce antibodies against the specific antigen.