Changes

Exposure to antigen causes CD4+ T helper cells to be activated leading to colonal expansion (takes 1-2 weeks). On subsequent exposure with the same antigen sensitised CD4+ T helper cells secrete cytokines which attract and activate macrophages. The 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 CD4+ T helper cells to be activated leading to colonal expansion (takes 1-2 weeks). On subsequent exposure with the same antigen sensitised CD4+ T helper cells secrete cytokines which attract and activate macrophages. The 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.

==3 types:==

==2 types:==

===Contact===

===1. Contact===

* Involves simple chemicals (antigens) which bind to skin proteins:

* Involves simple chemicals (antigens) which bind to skin proteins:

** Nickle

** Nickle

| '''Pathogenesis'''

| '''Pathogenesis'''

| Type I

| Type I

| Type II

| Type IV

|-

|-

| '''Time'''

| '''Time'''

|}

|}

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===Granulomatous===

===2. 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: