User:Sbartl/sandbox

This is now my sandbox and I will write a article here. If you are looking for the Bio 300 Assignment 3 page, search for my username in "Everything" or follow the link provided in iLearn.

Placed in the Evolution Section of the Adaptive Immune System page. Back links to this page were created on the Fish page Immune System section (with added text) and the Chondrichthyes page Immune System section (with added text). The back link already exists on the Gnathostomata page and was created on the Recombination activating gene page.

Evolution
The adaptive immune system, which has been best studied in mammals, originated in a jawed fish approximately 500 million years ago. Most of the molecules, cells, tissues, and associated mechanisms of this system of defense are found cartilaginous fishes. Lymphocyte receptors, Ig and TCR, are found in all jawed vertebrates. The most ancient Ig class, IgM, is membrane-bound and then secreted upon stimulation of cartilaginous fish B cells. Another isotype, shark IgW, is related to mammalian IgD. TCRs, both α/β and γ/δ, are found in all animals from gnathostomes to mammals. The organization of gene segments that undergo gene rearrangment differs in cartilaginous fishes, which have a cluster form as compared to the translocon form in bony fish to mammals. Like TCR and Ig, the MHC is found only in jawed vertebrates. Genes involved in antigen processing and presentation, as well as the class I and class II genes, are closely linked within the MHC of almost all studied species.

Lymphoid cells can be identified in some pre-vertebrate deuterostomes (i.e. sea urchins). These bind antigen with pattern recognition receptors (PRRs) of the innate immune system. In jawless fishes, two subsets of lymphocytes use variable lymphocyte receptors (VLRs) for antigen binding. Diversity is generated by a cytosine deaminase mediated rearrangement of LRR-based DNA segments. There is no evidence for the recombination activating genes (RAGs) that rearrange Ig and TCR gene segments in jawed vertebrates. The evolution of the AIS, based on Ig, TCR, and MHC molecules, is thought to have arisen from two major evolutionary events: the transfer of the RAG transposon (possibly of viral origin) and two whole genome duplications. Though the molecules of the AIS are well-conserved, they are also rapidly evolving. Yet, a comparative approach finds that many features are quite uniform across taxa. All the major features of the AIS arose early and quickly. Jawless fishes have a different AIS that relies on gene rearrangement to generate diversity but has little else in common with the jawed vertebrate AIS. The innate immune system, which has an important role in AIS activation, is the most important defense system of invertebrates and plants.