hypersensitivity reactions are excessive immune responses to normally harmless substances that induces inflammation and can lead to tissue damage and cell death there are four types of hypersensitive disorders in type 4 reactions these differ from the other three reactions in a couple of ways number one they are delayed reactions which means they can take days to even years to manifest therefore in some cases type 4 reactions can be known as delayed hypersensitivity another difference is the immune cell that they use are t cells rather than antibodies that we saw in the other three types therefore another name that's commonly used for type 4 is cell mediated hypersensitive disorders now in type 4 reactions these are comprised of a spectrum of severity from mild to severe so here i've broken type 4 reactions into two categories delayed reactions which usually just involve t helper cells and cytotoxic reactions which use both t helper cells and cytotoxic t cells now let's start with a delayed reaction this usually involves the skin only and where we have the antigen then enters some examples of the antigens can be latex can be metals like nickel or certain cosmetics when the antigen enters the skin it is taken up by antigen presenting cells such as a dendritic cell which then engulfs it here it then takes it back to a neighboring lymph node which then sensitizes and activates a t helper cell which then comes to the site on further exposure the t helper cell releases a whole lot of cytokines which then induces inflammation in the area this inflammation then leads to dermatitis rashes itchiness etc in cytotoxic reaction the same principle occurs the antigen is engulfed by an antigen presenting cell and presented to a t helper cell which then activates a cytotoxic t cell now where the cell with the antigen in some cases could be a cancer cell or a virally infected cell is then destroyed by the cytotoxic t cell which is a good thing but in some cases certain antigens which is of your own tissue in one case myelin can be seen as foreign therefore the same process occurs and cells that have myelin destroyed this disease is known as multiple sclerosis in another example the foreign antigen is seen to be on parts of beta cells beta cells aren't located in the pancreas that produces insulin so in the same process beta cells are destroyed in the pancreas leads to a disease known as type 1 diabetes