As in the case for B cell biology, it has become increasingly evident that complement also plays an important role in T cell biology. It affects T cell activation, T cell responses and in induction of regulatory T cells (for a review, see Kemper and Atkinson 2007) . For example, during T cell activation, absence of DAF on T cells and on antigen-presenting cells (APC) enhances T cell proliferation (Heeger et al. 2005) . Furthermore, APCs and T cells express C3aR and C5aR and locally produced C3a and C5a interact with their respective receptors and participate in maintaining T cell viability, proliferation and differentiation Lalli et al. 2008) .