BTK and Bruton-type agammaglobulinemia: Upstream of BTK, Fc receptors are known to interact with complement factors such as C5a (Schmidt and Gessner 2005; Karsten and Kohl 2012) and reduced Fc-, CR1- and CR3-mediated phagocytosis has also been reported in monocytes derived from XLA patients (Amoras et al. 2003).