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Lymphoid Cells and Organs- Evolutionary comparisons

Posted by Mumtaz khan Friday, 30 December 2011

         While innate systems of immunity are seen in invertebrates and even in plants,the evolution of lymphoid cells and organs evolved only in the phylum Vertebrata.Consequently,adaptive immunity,which is mediated by antibodies and T cells,is only seen in this phylum.

As one considers the spectrum from the earliest vertebrates,the jawless fishes(agnatha),to the birds and mammals,evolution has added organs and tissues with immune functions but has tended to retain those evolved by earlier orders.While all have gut-associated tissue (GALT)and most have some version of a spleen and thymus,not all have the ability to form germi centers is not shared by all.The differences seen at the level of organs and the tissues are also reflected at the cellular level.Lymphocytes that express antigen specific receptors on their surfaces are necessary to mount an adaptive immune response in lampreys and hagfish,members of order Agnatha,have failed.In fact,only jawed vertebrates ,of which the cartilaginous fish(sharks,rays)are the earliest example,have b and T lymphocytes and support adaptive immune responses.


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