ABSTRACT
C ontact hypersensitivity (CHS) reactions are prototypic delayed type hypersensitivity (DTH) reactions. They are mediated by interferon (IFN)-y-producing CD 4+ and C D 8 +, which are called type 1 T cells. Type 1 T cells can lead to the development of CHS reactions if directed against haptens or to autoimmune diseases when reacting with self anti gens. 1' 3 In humans, CHS reactions are T cell mediated skin diseases that display a heteroge neous inflammatory pattern depending on the nature of the contact allergen, the vehicle, and, most importandy, on the T cell and leukocyte subsets involved. 4 ' 6 Consequendy, the nature of murine and human CHS reactions as well as the underlying mechanisms are not identical, but share multiple similarities. 1 ’5 The most important common feature is the dependence on infil trating T cells. 1’2 ,5 ’7 These effector T cells can direcdy mount destructive effector functions when activated in the skin. 5 ’8 To induce the full clinical picture of inflammation type 1 effector T cells have to attract and activate other leukocytes or resident cells within the peripheral tissue. 2 Several types of leukocytes are involved in extensive inflammation, but the induction of specific T cells that emigrate into the tissues is a necessary prerequisite for the accumulation of the different leukocytes. The exact orchestration leading to the recruitment and activation of the different cell populations in the skin is still unclear. Moreover, the initial events leading to extravasation of type 1 T cells into skin remained enigmatic. Some studies found an important role for a B cell product in CHS reactions, despite the obligatory role of T cells, 9 and morpho logic studies suggested that activated mast cells (MC) play a role in human CHS reactions. 1 0 ’ 11 Recent findings now demonstrate that M C play a critical role both in the initiation and the amplification of CHS reactions. M C release the initial acute mediators opening the way for type 1 T cells to enter the skin and MC-derived factors later attract leukocytes like polymor phonuclear granulocytes (PMN) in response toT cell-derived signals. 2 ’ 12 ' 15 Thus, M C provide the link for the B cell-dependent T cell recruitment and subsequendy, M C respond to T cells during C H S . 2 ’9 ’ 15
This chapter will present some of these most recent data on the role of M C in CH SR and discuss mechanisms, relevance, and some consequences of these findings.
