Virtual Medical Centre

Hypersensitivity Reaction Type IV

• What is Hypersensitivity Reaction Type IV?
• Who gets Hypersensitivity Reaction Type IV?
• Predisposing Factors
• Progression
• Probable Outcomes
• How is Hypersensitivity Reaction Type IV Diagnosed?
• How is Hypersensitivity Reaction Type IV treated?
• Hypersensitivity Reaction Type IV References

What is Hypersensitivity Reaction Type IV?

Hypersensitivity means that the body responds to a particular substance (called allergens) in an exaggerated fashion, where it does not happen in normal circumstances. There are 4 types of hypersensitivity reaction, type I, II, III and IV. They are different in terms of the disease manifestation and pathological processes. However, only type IV hypersensitivity reaction will be discussed here.

Type IV hypersensitivity reaction is also called delayed type hypersensitivity reaction because it takes 2 to 3 days for the reaction to develop after exposure to the particular substance.

Type IV hypersensitivity reaction can occur in many parts of the body. Generally, they include:

• Skin: Atopic dermatitis
• Lungs: Tuberculosis , hypersensitivity pneumonitis, Wegener's granulomatosis
• Pancreas: Type I diabetes mellitus, or known previously as insulin-dependent diabetes mellitus.
• Thyroid: inflammation of the thyroid
• Other: transplant rejection, leprosy

The image below is that of a reaction to the allergen latex:

Who gets Hypersensitivity Reaction Type IV?

Type IV hypersensitivity diseases are common, but less common than type 1 hypersensitivity reaction diseases.

Predisposing Factors

Depending on the manifestations of different type IV hypersensitivity diseases, there are different risk factors as well.

Some diseases are more common in women such as autoimmune diseases. Some are more common in men such as allergic contact dermatitis. The exact mechanism of such discrepancies is unknown.

Likewise, ethnic and geographical differences have roles to play in different diseases.

Progression

Type IV hypersensitivity is characterized by cell-mediated response rather than antibodies as in other types of hypersensitivity reactions. Specifically, the T lymphocytes are involved in the development of the sensitivity, hence called cell-mediated hypersensitivity. T lymphocytes are white blood cells in the body.

After exposure to antigens, through a series of biochemical events, the T-cells will be activated. By releasing some chemicals, the T cells activate other white blood cells to mount an immune response.

This entire cascade of reactions take 2-3 days to develop and is different from other types of hypersensitivity reactions.