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** Cytotoxic T lymphocytes (CTLs) kill healthy cells mistakenly thinking they are infected by a virus.
** Cytotoxic T lymphocytes (CTLs) kill healthy cells mistakenly thinking they are infected by a virus.
−
===3 types:===
+
==3 types:==
−
====Contact====
+
===Contact===
* Involves simple chemicals (antigens) which bind to skin proteins:
* Involves simple chemicals (antigens) which bind to skin proteins:
** Nickle
** Nickle
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−
====Granulomatous====
+
===Granulomatous===
* Intense activation of T cells, cytokine release and macrophage activation cause necrosis of surrounding tissue, granuloma formation leading to destruction of host tissue.
* Intense activation of T cells, cytokine release and macrophage activation cause necrosis of surrounding tissue, granuloma formation leading to destruction of host tissue.
* Composed of epithelial cell, giant cells and macrophages in response to infection, for example:
* Composed of epithelial cell, giant cells and macrophages in response to infection, for example:
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** Brain/liver: very severe
** Brain/liver: very severe
−
====Tuberculin test====
+
===Tuberculin test===
* Injection of intradermal antigen into the skin.
* Injection of intradermal antigen into the skin.
* A skin reaction (infiltration of lymphocytes and monocytes) peaking at 48-72 hours indicates prior exposure to the antigen or ongoing infection.
* A skin reaction (infiltration of lymphocytes and monocytes) peaking at 48-72 hours indicates prior exposure to the antigen or ongoing infection.
* It is used for TB testing cattle; the antigen is purified protein derivative-PPD from ''Mycobacterium tuberculosis''.
* It is used for TB testing cattle; the antigen is purified protein derivative-PPD from ''Mycobacterium tuberculosis''.
* Presently in the UK the animal is culled if it has a positive skin reaction.
* Presently in the UK the animal is culled if it has a positive skin reaction.