ES-62 is a major secreted glycoprotein of the rodent filarial nematode Acanthocheilonema viteae and homologue of molecules found in filarial nematodes which parasitise humans. The molecule consists of a tetramer of apparently identical monomers of ∼62 kDa which we have shown by sedimentation equilibrium analytical ultracentrifugation to strongly associate. ES-62 is one of several filarial nematode proteins to contain the unusual posttranslational modification of phosphorylcholine (PC) addition. Specifically, we have found that PC is attached to one of three distinct N-type glycans we have characterised on the molecule. The amino acid sequence of ES-62 shows 37-39% identity with a family of 6 other proteins, some of which have been predicted to be amino- or carboxy-peptidases. We have also found that ES-62 is able to interact with a number of cells of the immune system, specifically B- and Tlymphocytes, macrophages and dendritic cells. Lymphocytes exposed to ES-62 in vitro or in vivo are less able to proliferate in response to ligation via the antigen receptor. Peritoneal macrophages pre-exposed to the molecule are less able to produce the cytokines IL-12, IL-6 and TNF-α following subsequent incubation with the classical stimulators IFNγ and LPS. Dendritic cells allowed to mature in the presence of ES-62 acquire a phenotype, which allows them to induce anti-inflammatory “TH2-type” responses. With respect to immunomodulation, the PC moiety of the parasite molecule appears to be predominantly responsible for the effects on lymphocyte proliferation at least and we have also found that its removal converts the murine IgG antibody response to ES-62 from solely IgG1 to mixed IgG1 / IgG2a. ES-62 appears to interact with cells of the immune system in a PC-dependent manner and, at least in part, via a molecule of ∼82 kDa. Studies of the interaction in lymphocytes show that it is associated with activation of certain signal transduction molecules including a number of protein tyrosine kinases and mitogen activated protein kinases (MAPkinases). Although such activation is insufficient to induce proliferation, it serves to almost completely desensitise the cells to antigenreceptor ligation-induced activation of the phosphoinositide 3-kinase (PI-3-kinase) and Ras / MAPkinase pathways, events critical for lymphocyte proliferation. Such desensitisation reflects ES-62-primed recruitment of a number of negative regulators of these pathways, such as the phosphatases SHP-1 and Pac-1.