Hypersensitivity reactions - Type IV: Nursing

A hypersensitivity reaction is an overreaction to a foreign antigen or against its own tissue 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 “cell mediated reaction” is not dependent on antibodies, which makes it different from the other 3 types. It’s called a delayed reaction because it usually takes 48 to 72 hours to recruit enough T cells and cause the reaction.

Let’s start by discussing the physiology of T cells. T cell precursors originate in the bone marrow but they migrate to the Thymus to mature; which is why they’re called T cells. In the thymus, they differentiate into many different types of T cells; this include T helper cells, and cytotoxic T cells, or killer T cells. Let’s talk about helper T cells first.

When a pathogen enters the body, 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 naive T helper cells which have never been exposed to antigens. The T helper cells will recognize the antigen with their T cell receptors or TCRs, and mature. They will then release cytokines that enhance the immune response. One of these is interleukin 2 which causes the T cell and other T cells to proliferate. Other cytokines can attract immune cells like macrophages and neutrophils to the area, and they can also cause the blood vessel walls to become leaky, allowing these additional immune cells to reach the tissue where that pathogen is located.