About 31% of the patients with a factor VIII replacement therapy develop a factor VIII inhibitor.From these are 23% low-responder ( 5BE) [8].In the case of severe hemophilia B,about 10.5% of the patients develop inhibitory antibodies [9]. Anti-factor VIII-antibodies are also seen in 15-78% healthy people without hemophilia [7, 17, 19]. Lacroix-Desmazes et al. [10, 11] showed anti-idiotypic antibodies neutralizing the inhibitory activity of the an- factor VIII antibodies in healthy people. Well-known predisposing factors for inhibitor formation are genetic features of factor VIII,which include large deletions,nonsense mutations or intrachromosomal recombinations [5, 23].Also, ethnic groups other than Caucasians (e.g.Africans) have a higher risk of developing inhibitors.Other risk factors are presumably de- ved from the immune system. For instance, a reduction of the inhibitor was seen with lower CD4+ T helper cell counts in HIV positive hemophilic patients [3,4].The development of inhibitors is very likely to be a Th-2 mediated event where cyto- nes and their receptors,T-cell receptors and the Major Histocompatibility Complex may also play an important role. Theoretical Background The substituted factor is an unknown protein for patients with a severe he- philia. Fig. 1. The normal immunoresponse (according to BAENKLER [2]) Abbreviations: TCR - T cell-receptor; APC - antigen presenting cell 36 I.Wieland et al.

This book contains the contribution to the 36th Hemophilia Symposium, Hamburg 2005. The main topics are epidemiolgy, hemophilia therapy, orthopedic treatment in hemophiliacs, hemostaseologic diagnosis and pediatric hemostaseology. The volume is rounded off by numerous free papers and posters on hemophilia, inhibitors in hemophilia and diagnostics.