how the parasitic worm protect themselves in the body of their host
Answers
Parasitic worms have several ways in which they manage to stay in good shape in face of immune response of a host. We can divide their strategy into four groups:
Parasites can locate themselves in places where immune responses are likely to be weak or ineffectual.
Parasites can repair damage inflicted upon them quickly enough to remain viable.
Parasites can harden themselves and become relatively insensitive to mechanism of immune responses.
Parasites can direct immune response towards ineffectual pathways.
Parasites can complete their reproduction before immune response creates problems for them.
Quite often it would be a combination of two or three of these strategies.
Parasites often hide inside the cells. While this is usually done by protozoan parasites, some worms can do that as well. Adult Trichinella spiralis females invade intestinal wall and make home in a number of adjacent cells of intestinal epithelium, by breaking through the lateral cell membranes. The larvae released by females migrate to the muscles, and take residence inside muscle cells, where they can live for decades.
Worms can also live within tissues, where immune system has limitations - for example, the full power of inflammatory response cannot be unleashed when the target is within fragile tissue, such as liver - the tissue would fall apart. This sometimes happens with microbial infection - for example, people with active tuberculosis may literally cough their lung tissue out. With worms, inflammation is usually limited in scope.
Parasites can repair the damage. Schistosomes, or the blood flukes, have multi-layer surface, which is continuously renewed - as the outer layer falls apart, new ones form underneath.
Some parasites are very difficult, hardened targets. Roundworms have a strong cuticle with a waxy surface - and this is something that immune system cannot do much about. The larvae of Echinococcus tapeworm live inside organs, within the bladders made of strong membrane.
Parasites can hide behind decoy antigens - they may release compounds that stimulate vigorous immune responses, but do nothing about a parasite itself. As parasitologists say, parasites are excellent immunologists - at least in practice… They survived for millions of years, in a continuous arms race with their hosts - and are quite good in exploiting weaknesses and deficiencies of immune responses. Sometimes they play nice - and avoid making an immune system upset. If getting rid of them is not worth the effort, the host organism may just forget the whole thing.
Parasites can also steer immune responses towards mechanisms that will not be very damaging to them, or will be more damaging to the host than to parasites. In these situations, the body may also become tolerant to a parasite.
Parasites may be quick in mating and reproducing. In many intestinal infections with roundworms, the worms mate and lay eggs before the immune response develops. Once it does, the worms are expelled from the intestine - but so what? This, however, may limit the parasitic burden in host that were exposed to the parasite previously, so it is not necessarily without any effect on a parasite.
What I presented here is just a glimpse on the very complex and complicated interaction. These issues are far from being well understood, especially in view of our limited understanding of the intricacies of the immune system itself.