Changes

==Introduction==

==Introduction==

[[Image:Sensitisation phase Type IV Hypersensitivity.jpg|right|thumb|150px|Sensitisation phase: Type VI hypersensitivity-Brian Catchpole RVC 2008]]

[[Image:Sensitisation phase Type IV Hypersensitivity.jpg|right|thumb|150px|Sensitisation phase: Type IV hypersensitivity-Brian Catchpole RVC 2008]]

[[Image:Delayed type hypersensitivity.jpg|right|thumb|150px|Type VI hypersensitivity-Brian Catchpole RVC 2008]]

[[Image:Delayed type hypersensitivity.jpg|right|thumb|150px|Type IV hypersensitivity-Brian Catchpole RVC 2008]]

Cell mediated hypersensitivity:

Cell mediated hypersensitivity:

Type IV hypersensitivity also known as delayed hypersensitivity involves [[Cytokines - WikiBlood|cytokines]] being secreted from the Th-I cells, which cause the activation of [[Macrophages - WikiBlood|macrophages]] and other T cells.

Type IV hypersensitivity, also known as delayed hypersensitivity, involves [[Cytokines|cytokines]] being secreted from the Th-I cells, which causes the activation of [[Macrophages|macrophages]] and other T cells.

* [[Lymphocytes - WikiBlood#Helper CD4+|'''CD4+ (helper)''']] mediated:

* [[Lymphocytes#Helper CD4+|'''CD4+ (helper)''']] mediated:

** Abnormal activation of macrophages in healthy tissues.

** Abnormal activation of macrophages in healthy tissues.

** Macrophage production of inflammatory mediators and MMP (matrix metalloproteinase) enzymes cause tissue damage.

** Macrophage production of inflammatory mediators and MMP (matrix metalloproteinase) enzymes cause tissue damage.

* [[Lymphocytes - WikiBlood#Cytotoxic CD8+|'''CD8+ (cytotoxic)''']] mediated:

* [[Lymphocytes#Cytotoxic CD8+|'''CD8+ (cytotoxic)''']] mediated:

** 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.

Exposure to antigen causes [[Lymphocytes - WikiBlood#Helper CD4+|'''CD4+ T helper cells''']] to be activated leading to colonal expansion (takes 1-2 weeks). On subsequent exposure with the same antigen sensitised [[Lymphocytes - WikiBlood#Helper CD4+|'''CD4+ T helper cells''']] secrete [[Cytokines - WikiBlood|cytokines]] which attract and activate macrophages. The [[Macrophages - WikiBlood|macrophages]] have an increased ability to phagocytose pathogens, which is very important for the clearance of intracellular pathogens. However if antigen exposure persists, the lytic products of the macrophages can damage healthy tissues.

Exposure to antigen causes [[Lymphocytes#Helper CD4+|'''CD4+ T helper cells''']] to be activated leading to colonal expansion (takes 1-2 weeks). On subsequent exposure with the same antigen sensitised [[Lymphocytes#Helper CD4+|'''CD4+ T helper cells''']] secrete [[Cytokines|cytokines]] which attract and activate macrophages. The [[Macrophages|macrophages]] have an increased ability to phagocytose pathogens, which is very important for the clearance of intracellular pathogens. However if antigen exposure persists, the lytic products of the macrophages can damage healthy tissues.

==2 types:==

==2 types:==

{| style="width:60%; height:200px" border="1" align=left

{| style="width:60%; height:200px" border="1" align=left

!

!

![[Allergy - WikiBlood#1. Atopic dermatitis - Dogs and horses|'''Atopic dermatitis''']]

![[Atopic Dermatitis|'''Atopic dermatitis''']]

!'''Contact dermatitis'''

!'''Contact dermatitis'''

|-  

|-  

| '''Pathogenesis'''

| '''Pathogenesis'''

| [[Type I Hypersensitivity - WikiBlood|Type I]]

| [[Type I Hypersensitivity|Type I]]

| [[Type IV Hypersensitivity - WikiBlood|Type IV]]

| [[Type IV Hypersensitivity|Type IV]]

|-

|-

| '''Time'''

| '''Time'''

|-

|-

|'''Pathology'''

|'''Pathology'''

| [[Mast Cells - WikiBlood|mast cells]], [[Eosinophils - WikiBlood|eosinophils]]

| [[Mast Cells|mast cells]], [[Eosinophils|eosinophils]]

| [[Monocytes - WikiBlood|mononuclear cells]]

| [[Monocytes|mononuclear cells]]

|}

|}

<Br clear="left">

<Br clear="left">

===2. Granulomatous===

===2. Granulomatous===

* Intense activation of T cells, [[Cytokines - WikiBlood|cytokine]] release and [[Macrophages - WikiBlood|macrophage]] activation cause necrosis of surrounding tissue, granuloma formation leading to destruction of host tissue.

* Intense activation of T cells, [[Cytokines|cytokine]] release and [[Macrophages|macrophage]] activation cause necrosis of surrounding tissue, granuloma formation leading to destruction of host tissue.

* Composed of epithelial cell, [[Macrophages - WikiBlood#Giant cells|giant cells]] and [[Macrophages - WikiBlood|macrophage]] in response to infection, for example:

* Composed of epithelial cell, [[Macrophages#Giant cells|giant cells]] and [[Macrophages|macrophage]] in response to infection, for example:

** [[Mycobacteria spp.|''Mycobacterium tubercle'']]

** [[:Category:Mycobacterium species|''Mycobacterium tubercle'']]

** Schistosome eggs

** Schistosome eggs

* Granulomas can obstruct normal organ function:

* Granulomas can obstruct normal organ function:

===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 [[Mycobacteria spp.|''Mycobacterium tuberculosis'']].

* It is used for TB testing cattle; the antigen is purified protein derivative-PPD from [[:Category:Mycobacterium species|''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.