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Cell-mediated hypersensitivity is a delayed reaction. This delayed reaction is due to the function of T Lymphocytes, not antibodies. The immune system responds to an antigen exposure which can take a day or more to develop. It can be transferred by immunologically T cells. There are three subtypes of Type IV hypersensitivity: Contact dermatitis, tuberculin-type hypersensitivity, and granulomatous-type hypersensitivity. An example of contact dermatitis is exposure to touching poison ivy.

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Poison ivy reactions occur as a result of contact with one of three types of plants: poison ivy, poison oak, and poison sumac.
Approximately eighty-five percent of people will develop an allergic reaction to these plants.

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The signs exhibited of this type of hypersensitivity include a local inflammatory reaction. The symptoms are erythema, warmth, edema, and fever. Chronic inflammation can lead to granuloma formation on the skin which can lead to organ failure. A patch test can be performed to analyze the allergen. A topical corticosteriod can be prescribed to control the inflammation.

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The use of certain antibiotic and anticonvulsant drugs can trigger type IV hypersensitivity and lead to drug hypersensitivity and clinical disorders.

Type IV hypersensitivity can also be triggered when viral infections such as cytomegalovirus is exposed to some antibiotics, or when Epstein Barr virus is exposed to amoxicillin, and herpes6virus exposed to anticonvulsants.

The use or exposure to latex gloves can also trigger type IV hypersensitivity.

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If patient has been infected with contact dermatitis from poison ivy, the first thing to do is to remove the offending agent (the poison ivy). If the situation is still severe after the removal of the offending agent, use topical steroids.

If the patient is suffering from Steven Johnson syndrome or toxic epidermolysis, patient should be admitted in the ICU and given optimal fluid therapy and antibiotics if there is a secondary infection.

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Regulatory T cells in Multiple Sclerosis 

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Regulatory T cells in Multiple Sclerosis 

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Myelin specific T-cells release cytokines that then dilate blood vessels allowing more immune cells to pass the blood brain barrier thus producing more inflammation and damage to oligodendrocytes.  In the animation depicted above are the policing capacities of cell mediated type IV hypersensitivity reactions.  Specific regulatory T cells are able to pass the blood brain barrier in autoimmune attacks to inhibit other immune cells thus causing less inflammation. Over time these capacities are overwhelmed to cumulative damage therefore these policing functions become inhibited.

Animation via
https://www.youtube.com/watch?v=yzH8ul5PSZ8

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Multiple sclerosis - causes, symptoms, diagnosis, treatment, pathology

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Watch and share Multiple Sclerosis GIFs and Kkhalios GIFs on Gfycat

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If the patient is allergic to poison ivy, the initial reaction will make reactive T cells, which won't cause a visible reaction to the site. However, if the patient comes in contact with Poison Ivy again, it will activate the type IV hypersensitivity reaction causing a visible dermatitis reaction.

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T - Cell Mediated Hypersensitivity Type IV

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T - Cell Mediated Hypersensitivity Type IV

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T - Cell depicted: CD4 + T-Helper cells, they release cytokines locally that can stimulate or inhibit immune cells in vicinity.

https://fb.watch/jnmhLv04ES/

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Type IV hypersensitivity (cell-mediated) - causes, symptoms, treatment & pathology

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Pathophysiology at the Tissue/Organ Level

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Pathophysiology at the Tissue/Organ Level

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As previously mentioned, an individual will not have a visible reaction to poison ivy on the first contact; however, reactive T cells in the immune system are able to recognize the oils of the plant and cause a dermatitis reaction.
Poison ivy primarily affects the integument system, as a dermatitis reaction occurs. Type 4 sensitivity reactions invoke the involvement of T-cells in order to initiate a response, so the immune system also plays a significant role as well.
The rash caused by a poison ivy reaction can occur wherever an individual's skin came into contact with the oils of the plant, but it can also be spread further on the skin.

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The Coombs and Gel's classification essentially categorizes allergies into four pathophysiological types. Depicted above is type IV: Cell Mediated hypersensitivity. Defined by the delayed hypersensitivity reaction. Standard onset of symptoms to develop in type IV reactions is around 12 hours. Although, maximal reaction time can reach anywhere between 48 to 72 hours.
Justiz Vaillant AA, Zulfiqar H, Ramphul K. Delayed Hypersensitivity Reactions. [Updated 2022 Sep 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-.
Available from: https://www.ncbi.nlm.nih.gov/books/NBK519023/

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Type-IV Hypersensitivity: Delayed Type on Make a GIF

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Type 4 Hypersensitivity: Poison Ivy Reaction Conforms to The Traditional Definition

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Type 4 Hypersensitivity: Poison Ivy Reaction Conforms to The Traditional Definition

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Type 4 reactions are cell-mediated, which means the response is controlled by T cells rather than antibodies. Poison Ivy leaves are coated with chemicals called urushiol. If urushiol touches the skin, it causes allergic contact dermatitis. In allergic contact dermatitis, the carrier protein is in the skin. This is a T cell-mediated response, also known as delayed hypersensitivity. The body's immune system attacks the complex of urushiol derivatives with skin proteins. Macrophages become activated by cytokines, causing tissue damage. The response is delayed, occurring 24-72 hours after antigen reexposure.

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Type IV Hypersensitivity (Described Concisely)

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Development of Allergic Contact Dermatitis

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Development of Allergic Contact Dermatitis

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In contact dermatitis, the carrier protein is in the skin. An exemplar condition is a reaction to poison ivy. The urushiol antigen reacts with normal skin proteins and causes a cell-mediated immune response. Primary contact with an allergen produces reactive T cells. As a result, the person in contact does not have dermatitis. Secondary contact activates type 4 cell-mediated hypersensitivity reaction that causes dermatitis.