Parasitic Flatworms

.. and in vitro. The host molecules acquired by the schistosomes were in fact surface components of the erythrocyte; A, B, H, And Lewis b+ antigens were acquired by parasitic flatworm. Even more interesting was the fact that A and B antigens could be acquired from the serum of A or B positive donors in the absence of homologous erythrocytes, irrespective of the secretor status of the donor. This provided information that the blood group substances were taken up as glycolipids rather then glycoproteins.

This proof was derived from an experiment done by Goldring, Kusel and Smithers ( as cited in Parasitology 1988 ) as mentioned by D. J McLaren. Schistosomula grown in vitro with a megalolipid extract of the A blood group antigen expressed A antigen on their surfaces and secondly, erythrocytes whose surface carbohydrates were radio-isotope labeled were found to transfer only labeled glycolipid like molecules to the surface of co-cultured Schistosomula. (p.599). The molecular interdigitation of the glycolipid with the parasites tegumental outer membrane, to leave the haptenic carbohydrate portion of the molecule exposed is another view of the acquired host antigens with the parasite surface.

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Proof of such association is evident in other experiments done. It has been shown that, Lewis blood group glycolipids have been shown to transfer from serum to co-cultured deficient erythrocytes ( D. J. McLaren 1988, p.599 ). Histocompatibility Agents An additional way that the parasite evades the immune response is by the uptake of other host molecules taken up as glycoproteins. As described, The existence of contaminating antigens of host origin in parasites.. made it necessary to introduce and define a new term eclipsed antigen for an antigenic determinant of parasite origin which resembles an antigenic determinant of its host ( Damien 1964 p.130 ).

Therefore the host will not be able to recognize a parasite as foreign, thus not producing antibodies to evoke an attack. It has been shown that Schistosomula posses serologically detectable alloantigens on their surface: the major determinants of immunological recognition of self. Gene products of the K, D and Ia regions of the MHC were demonstrated by experimental techniques. ( D.J Mclaren, 1988 )These MHC-coded antigens were further shown to be acquired in vitro following co-culture of lung stage parasites with allogeneic lymphocytes, and that reinjection in vivo , using these allogeneic recipients showed that an exchange of the acquired alloantigens can be exchanged within 87 hrs. Demonstrating that the MHC antigens can be acquired by lung stage Schistosomula following culture in the presence of human platelets.

Its has also been noted that MHC-encode alloantigens were detected on the surface of older larvae ( 21 days ), and adult worms. Thus showing that he alloantigens can be acquired through various stages of the Schistosomula stages of life. Evidence gathered has shown that the schistosome can acquire the MHC gene products from the host and does not synthesize them itself. Information gathered from researchers such as, Simpson, Singer, McCutchan, Sacks and Sher, have shown that there is DNA sequences in the parasite genome homologous to class MHC antigens. It is also notable to state here that certain regions of the MHC are known to selectively shed, in association with membrane lipids.

These host lipids are known to associate readily with the schistosomular surface, a mechanism for the transfer of MHC antigens to the parasite can thus be envisioned. (D.J McLaren 1988, p.601) Intracellular Substance Agents Intracellular substance antigens, with specifications confined to the tegument of certain skin cells, have also been detected on the surface of skin-penetrated schistosomlua. These antigens were not present when cercariae were transported by mechanical means into the host, nor were they present in Schistosomula recovered from the lung of an animal on day 5. This information brings about some doubt as to the long term value of the intracellular substance antigens in the disguise process. It is perhaps a characteristic used only to evade and gain entrance into the host, and once within, the parasite loses this antigen. immunoglobulins An area of interest to numerous researchers is the acquisition of host immunoglobulin by the parasite.

In schistosomes it has been noted that there was a presence of IgG1, IgG2a, IgG2b, IgG3, IgA, and IgM on the surface of the worm. It has been shown that these antibodies are to be hetrospecific rather than the classical blocking antibodies. They are also noted to be bound to the surface of the parasite via Fc receptors on the tegumental outer membrane. However in adults, experiments done by Torpier, Carpon & Ouaissi have shown that ( as cited in Parasitology 1988 ) Rosetting techniques have failed to confirm the presence of Fc receptors on adult schistosomes…Fc with human receptors with specificity to human b2-microglobulin were detected on the surface of skin-penetrated Schistosomula using this technique (p.602) Thus giving the parasite a cloak against the immune response of the host. Protection According to Age of The Flatworm As it has been shown, a considerable amount of effort has been devoted to the location and identification of host molecules on the tegumental surface of the parasite. Even though most information gathered has not been able to conclusively prove that shared determinants serve as a disguise of the parasites flatworm to the host immune system, another alternative may be that may serve as a different but as important function.

It is conceivable that the parasite does synthesize a molecule that mimics a host immunoregulatory protein There has been a certain evidence that has shown that murine protein and the worm synthesized determinant share physicochemical and immunological characteristics..and function to inhibit T-cell induced lymphocyte blastogenesis ( D. J Mclaren, 1988 p.604 ). The stage at which the flatworm is in would seem apparently important as some experiments have shown. It seems that the older schistosome rely exclusively upon their disguise for protection against the immune response of the host, but the younger stages have additional mechanisms of protection, termed intrinsic This intrinsic mechanism reportedly in the young is shown to be of some interest. Resistance against immune attack has been reported by some authors to develop in the absence of host molecules, but the general consensus of opinion is that protection proceeds faster and more efficiently if the worms were given access to mammalian serum.

In this context the young schistosome has been shown to selectively incorporate host lipids and to be capable of performing a limiting range of interconversions. These changes in lipid composition have been correlated with increased protection against both complement mediated and eosinophil-mediated cytotoxicity. In other experiments it was shown that modulation of cell surface lipids were known to alter the susceptibility of the cell to complement lysis. Therefore it seems that the lipid exchange between the young schistosome and its host serves to secure the tegumental membrane and supplement the protection afforded by the acquired host antigen disguise. It has also been suggested that the immunoglobulin absorbed by schistosomes play an important role in membranes modulation. This was demonstrated when rabbit antibody was complexed to mouse immunoglobulin on the parasite surface, that particular immunoglobulin was shed from the tegumental outer membrane very quickly ( D.J McLaren 1988, p.607 ).

This suggests that when the outer membrane was damaged a quick turnover of the membrane was noticed. Effectively protecting it from further damage from the host immune system. SUMMARY The Balance We have taken a look at a few ways in which the flatworm parasite can evade its host immune system. It is obvious that the outer tegumental membrane of the parasite is a crucial element in the survival of the evasive flatworm. Strong evidence indicating that acquired blood group determinants and incorporation of host lipids help in this protection. Though the offense of the parasite may be strong, a balance between the host and parasite must be reached. Each host-parasite relationship is a unique product of the particular individual involved..there is therefore a complex trade off for both partners between the beneficial and harmful effects of the host response ( Wakelin 1993, p.

493 ) The interactions between the two will be dependent on one another, the complexity of the genetically determined response of the host immune system on the parasite, and the intricate strategies employed by the parasite. The parasite does not want to be terminated nor expelled by the immune response of the host, however to much taken from the host and the parasite finds itself in is situation where the host is incapable of providing nutrients for both the parasite and itself, thus destroying both individuals. A balance must be found for a successful existence, between the host and parasite. One could almost consider the interaction of the parasite and host to lead to coevolutionary arms race, in which an evolutionary progress in one side provokes a further response in the other side. The host should evolve defensive means to reduce the impact of paratism, while the parasite should evolve mens to counter the host defense.

Evasive techniques applied by humans It is impossible to avoid all situations that could lead to parasitic infection. There are however a few basic precautions that can serve as a guidelines to better protect oneself. Drinking pure clean water, always washing produce (especially vegetables), cooking meat thoroughly, and maintaining a healthy lifestyle that includes keeping fit. A strong body makes for a great line of defense against parasites.


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