الفهرس 
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Abstract
Nickel considered one of the most irritant materials that may cause delayed hypersensitivity reactions and widely used in orthodontic materials. The aim of our study was to detect the immunohistopathoogical changes of Nickel and Nickel free orthodontic brackets that had been implanted subcutaneously in rats.
Three types of brackets were used Nickel, silicon with metal slots and silicon brackets. Fifty rats were randomly divided into ten groups, the animals were sacrificed after one ,two and four months ,and they were examined to determine both cellular and humoral immune reactions, tissue samples were obtained from the site of implantation and spleen ,blood samples were also collected.
The rats which were implanted with nickel brackets and sacrificed after 1 and 2 months showed sever immune reaction at the site of implantation and spleen (cellular component of white and red blood). Neutrophils and lymphocytes of blood were also more or less involved in the immune reaction.
The degree of inflammatory reaction decreased by time. This was proved when the animals were sacrificed after 4 months. The animals which were implanted with silicon with metal slot brackets showed lesser degree of inflammatory
reaction and totally subsided after 4 months .The animals which were implanted with Nickel free brackets (silicon brackets) did not show any sign of inflammatory reaction.