What is the defence mechanism of phylum Nematoda?

Nematode parasites live in the digestive system of the host and so employ some mechanism to prevent being digested and prevent triggering an immune response.

For example, Ascaris roundworms secrete a series of inhibitors to target digestive and immune-related host proteases, which include pepsin, trypsin, chymotrypsin/elastase, cathepsins, and metallocarboxypeptidases (MCPs).

Ascaris inhibits MCPs by releasing a enzyme known as Ascaris carboxypeptidase inhibitor (ACI). This enzyme binds to the active site of MCP and blocks the cleavage of its own proteins by the host MCP