Parasite nucleotide-metabolizing enzymes and host purinergic signalling.

Tissue damage results in a variety of molecular signals that activate elements of the immune system. Recent years have seen a growing awareness that key regulators of these events are extracellular nucleotides that signal through purinergic receptors. Haematophagous insects and ticks secrete enzymes in their saliva that degrade nucleotides, thus inhibiting haemostasis and minimizing the ensuing pain and inflammatory reactions provoked by these mediators. The discovery of an enzymatic cascade of nucleotide-metabolizing enzymes in secreted products of Trichinella spiralis suggests that endoparasites use similar mechanisms to modulate host purinergic signalling.