In order to chronically infect their hosts, filarial nematodes have generated a range of strategies to evade and down-modulate the host's immune system. The recent concept of suppression of immune responses by regulatory T cells has in part benefited from examinations in human and murine filariasis. Its further development in basic immunology animal models has in turn helped to better understand down-regulatory immune mechanisms in filariasis. Thus, filarial nematodes orchestrate down-regulation by inducing regulatory T cells and alternatively activated macrophages, which are able to suppress both Th1 and Th2 responses. Regulatory T cells can also induce the secretion of IgG4 from B cells as another arm of modulation. Dendritic cells are down-regulated upon first encounter with infective L3 larvae. Failure to respond to down-regulatory induction is based on genetic traits in hosts and leads to reduced parasite loads, albeit at the expense of pathology and disease. Since down-regulation in chronically and heavily infected hosts extends to third-party antigens, it is essential to analyse the impact of filarial infection for vaccination, allergy and important coinfections such as malaria, in order to foresee and avert potentially disastrous consequences of filariasis control programmes.