Contact hypersensitivity

Contact hypersensitivity is a cutaneous reaction involving the dendritic cells of the skin - Langerhans cells. Contact allergens bind to particular amino acids on proteins. Thus, the allergen (which normally alone may not stimulate an immune response) becomes a hapten on a carrier protein. This hapten is then seen by the immune system as an antigenic determinant on the larger protein. Langerhans cells are able to process these haptenated proteins and present antigen to T cells in the local cutaneous lymph nodes. Once activated, the T cells produce cytokines and chemokines that encourage the migration and activation of local macrophages initiating a local, cutaneous inflammatory response. Critical in the induction and initiation of the response are IL-12 (from Langerhans cells) and IL-4, IL-10 and IFNg (from T cells).

Although considered to be a Type IV hypersensitivity response, contact hypersensitivity differs from classical DTH reactions in that it may be mediated by either CD4+, or CD8 + , T cells (DTH is usually CD4+ T cell-dependent). In addition, the contact response is not necessarily regulated by a Th1 response, as is the case with DTH reactions. However, much of our understanding of contact hypersensitivity comes from studies with knock-out mice and so we have yet to obtain a true understanding of the cellular and molecular basis of the reaction in humans.

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