Changes

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

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

==2 types:==

==2 types:==