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Disorders Involving Cell-mediated Immunity (Type IV Reactions) in Horses


Ian Rodney Tizard

, BVMS, BSc, PhD, DSc (Hons), DACVM, Department of Veterinary Pathobiology, College of Veterinary and Biomedical Sciences, Texas A&M University

Reviewed/Revised Mar 2019 | Modified Oct 2022

This type of reaction occurs when specific types of white blood cells (called T helper cells) respond to antigens and release toxic and inflammatory substances that can damage tissues. Cell-mediated immune reactions can occur in any organ. Treatment usually involves the use of anti-inflammatory drugs and drugs that suppress the immune system, either alone or in combination.

Granulomatous Reactions

Granulomatous reactions are masses of fibrous connective tissue infiltrated by the white blood cells that form a cell-mediated immune response. They occur in some animals following infection with certain types of bacteria or fungi. Although cell-mediated immune responses effectively fight off these infections in most individuals, in a few animals the immune response is only partially effective and results in a mass (called a granuloma) at the site of infection.

Contact Hypersensitivity

Contact hypersensitivity results from chemicals reacting with skin proteins. These reactions modify skin proteins in such a way that they are perceived as foreign invaders. The body then produces a cell-mediated immune response against them and causes skin damage. This hypersensitivity usually occurs as a result of contact with sensitizing chemicals, including some medications and insect repellents.

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