Since all these abacavir reacting TCC were generated from abacavir naı ¨ve individuals, the relevance of such cells in the pathogenesis of abacavir HR remains to be confirmed. DC maturation is considered the essential first step to generate a primary immune response to novel antigens. Nevertheless, the role of DC in initiating immune responses to drugs is unclear. Whereas DC maturation by haptens in contact hypersensitivity models has been well studied and confirmed, the source and the role of co-stimulation for other mechanisms involved in drug HR have been questioned. Without hapten formation, immune activation by drugs might bypass the innate immune system. The data obtained in this study with abacavir support this assumption. We did not find any evidence for abacavir-induced DC activation. Neither up-regulation of rather sensitive maturation markers nor secretion of pro-inflammatory cytokines was detected upon abacavir exposure. Since it is difficult to conclude from a missing signal to its absence, these findings were confirmed by the successful induction of abacavir-reacting T cells in the absence of monocytes or DC in the culture. Thus, our data argue against a need for DC activation in the primary response to abacavir. The fact that co-stimulatory signals are not required for the induction of abacavir reactivity suggests that abacavir-reacting T cells originate from the memory compartment. On second antigen exposure, memory T cells do not require further co-stimulation. For this reason we already hypothesized in previous reports, that p-i reacting T cells stemmed from the memory pool. The characteristics of abacavir-reacting T cells are shared by allo-reacting T cells. We had already reported that a high proportion of CD4 + T cells specific for lidocaine or sulfamethoxazole were allo-reactive. To our knowledge, this is the first time allo-reactivity of MHCclass I restricted drug-reacting CD8 + T cells is described. In this study, only HLA-B*58:01 was shown to be allostimulatory. Since only 5% of TCC were allo-reactive, alloreactivity could be detected at the TCC level but not at the TCL level. In these habitats, chitin, a polymer of N-acetylglucosamine and the second most abundant carbohydrate in nature, can constitute an important source of carbon and nitrogen. Many bacteria autochthonous to chitin-rich environments have developed simple to complex chitinolytic systems, which allow them to degrade chitin and use it as a nutrient source. Most importantly, chitin is broken down by chitinases, which are often assisted by lytic polysaccharide monooxygenases that facilitate chitinase accessibility to the complex crystalline chitin structure. Interestingly, many human and animal pathogens are chitinolytic, and evidence has emerged that chitinases and LMPOs have an additional key role in these pathogens as virulence factors, which promote infection.