Changes

** 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:

** 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.