It renders the parasite’s resistant to immune-effector mechanisms. Parasites may shed their antigenic coats, either spontaneously or after binding with specific host antibodies. Schistosoma larvae develop a tegument while travelling to lungs of infected animals that is resistant to damage by complement and cytotoxic T lymphocytes of the host.

Protozoan parasites may conceal themselves from the immune system by living inside the host cells or by developing cysts that are resistant to immune effectors of the host. Besides, complement and phagocytic cells are not found in intestine, thus some helminthic parasites are sheltered from cell-mediated immune mechanism of the host. Among different immunoglobulines, IgA is secreted into intestinal lumen which is not a potent effector molecule against worms that inhabit there. The particular location of the parasite within the host’s body may provide some protection against host defences. Here some examples of parasitic evasion are cited: Successful parasites display an astonishing array of tactics and evade protective immunity by reducing their immunogenicity and inhibiting host immune responses. The most important factors behind the successful establishment of parasites, especially endoparasites, is their ability to overcome the host’s internal defence mechanism.

(b) Overcoming the host’s defence mechanism: Such as, the tapeworm Echinococcus granulosus become embedded in its definitive host’s mucosal crypts with the scolex. Attachment not only must prevent involuntary shifting of the parasite from its habitat, but in some cases is essential for its physiologic well- being. In addition to occupying a compatible site, each parasite must become mechanically attached. Habitat selection is based on biochemical, physiological and physical needs. Likewise, some lice are found only at certain sites on host’s body. Adult Fasciola spp, usually occur in the bile ducts of their definitive host. Both ecto- and endoparasites are very specific in their habitat selection.

Parasite in city stage 3