Final answer:
In contact dermatitis, T-cells are activated when antigen presenting cells present allergen complexes with MHC I molecules to naïve CD8 T-cells, which differentiate into cytotoxic lymphocytes targeting 'self' cells. Additionally, memory CD4 TH1 cells release inflammatory cytokines to stimulate macrophages, exacerbating the response upon re-exposure to the allergen.
Step-by-step explanation:
In contact dermatitis, T-cells are activated by a process involving the recognition of foreign substances called allergens by the immune system. There are different subtypes of T-cell activation based on the nature of the allergen and the type of immune response they produce.
In the case of contact dermatitis like that caused by poison ivy, the allergen is often a small molecule that combines with proteins in the skin, forming a complex that is recognized as foreign. Local antigen presenting cells (APCs) phagocytose this complex and present it on their surface with MHC I molecules, activating naïve CD8 T-cells. These CD8 T-cells proliferate and differentiate into cytotoxic T lymphocytes (CTLs) which can then target and destroy any 'self' cells presenting the same allergen-MHC I complex, leading to tissue damage and the characteristic rash associated with contact dermatitis.
Another pathway involves CD4 TH1 cells that are activated by APCs during the sensitization phase. Upon re-exposure to the allergen, these memory T-cells release inflammatory cytokines that stimulate macrophages and further aggravate the inflammation and tissue damage. In the case of delayed-type hypersensitivities (DTH), like poison ivy, subsequent exposures lead to a more rapid and intense response, causing an inflammation that continues until the allergen is removed.
Contact dermatitis can also occur with substances such as nickel, where prior sensitization to the metal is necessary for an allergic reaction upon skin contact. However, the manifestation of symptoms in contact dermatitis can be influenced by both the T-cell mediated immune response and the toxicity of the substances involved.