Editing Hypersensitivity (section)

== Type IV hypersensitivity ==

{{Main|Type IV hypersensitivity}}

[[File:Type IV hypersensitivity.webm|thumb|Type IV hypersensitivity]]
Type IV hypersensitivity reactions are, to some extent, normal physiological events that help fight infections, and dysfunction in this system can predispose to multiple opportunistic infections. Adverse events can also occur due to these reactions when an undesirable interaction between the immune system and an allergen happens.<ref name=":3" />

=== Pathophysiology ===
A type IV hypersensitivity reaction is mediated by T cells that provoke an inflammatory reaction against exogenous or endogenous antigens. In certain situations, other cells, such as monocytes, eosinophils, and neutrophils, can be involved. After antigen exposure, an initial local immune and inflammatory response occurs that attracts leukocytes. The antigen engulfed by the macrophages and monocytes is presented to T cells, which then becomes sensitized and activated. These cells then release cytokines and chemokines, which can cause tissue damage and may result in illnesses.<ref name=":3" />

Examples of illnesses resulting from type IV hypersensitivity reactions include [[contact dermatitis]] and drug hypersensitivity. Type IV reactions are further subdivided into type IVa, IVb, IVc, and IVd based on the type of T cell (Th1, Th17, and CTLs) involved and the cytokines/chemokines produced.<ref name=":3" />

Delayed hypersensitivity plays a crucial role in our body's ability to fight various intracellular pathogens such as mycobacteria and fungi. They also play a principal role in tumor immunity and transplant rejection. Since patients with acquired immunodeficiency syndrome ([[HIV/AIDS|AIDS]]) have a progressive decline in the number of CD4 cells, they also have a defective type four hypersensitivity reaction.<ref name=":3" />