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Unformatted text preview: When we look at immunity, B cells play a role in both innate and acquired immunity. There are two knnown subsets. The one we know about first were the B1 or convention cells. B1 can be divided into two subsets. Some expressprotein CD5. DNK B 1 or B2. Just that sometimes they express and some times they dont' express CD5. They're very long lived and self replenishing. Fetal omentum, piece of tissue that hangs down from GI tract. They are not in the bone marrow after birth. They're from the peritoneal and pleural cavities. They are rare in spleen and absent from the lymph nodes. They're also unusual that they encode a very limited V region repertoire. They bind to antigens weakly such as phosphryolcholine. They're IgMs. They predominately produce specificities encoded in the germline. Annate. They're responsible for the majrority of nonimmune serum. B cells do n ot require T cell help. Abs are mostly germline cencoded. They can be very rapidly turned on. Germline encoded rapid response. They recognize patterns on antigens. It's hypothesized there'sa primitive type b cells. Much like the gamma or dleta t cells that predominate in the fetal mouse. They are abudnant in autoimmjune disease. They are weakly self-reacive. In contrast, B2 B cells are are the principle B cells in the secondary lymphoid organs. The stem cells continually generate and the principal b cells are responsiable for the adaptive immune response. They have an extenseive variable region repertoire which is different from the B1 B cell.s They can recognize almost any antigen. They are somatically generated. They require T cell help. And because of that, they show memory. B1B cells don't have memory. They also isotype switch which also requireds Tell help. There are two B2 cellsT dependent Ag response. MZ are more recently found. They are halfway between B1 and B2 B cells. As an aside, memory B cells can also be found in the marginal zone in the spleen. The ontogenhy of the convential B cells, the hematopoiesis takes place in the liver. It is the source of the stem cells. The initial Bell differentiation takes place in the bone marrow. This is the bone and marrow, the SC are found in the outside, as they divide, they migrate towards the large vein in the middle. The spongy matrixof reticular cells profice growth and diff factors. In the diagram, there are adhessiomn molecules on the proB cells. We'll go through this a couple times. The inital B cell has an adhesion molecule that raeacts with another adhesion molecule on the stromal cell. Then theremolecule cKIT that interacts with SCF causing them to divide and dif into pre B cell. The Pre B cell now exrpresses IL7 R. IRemember, the first isotype expressed is always IgM. I'm going to go through this table. A molecule that is characteristic of B cells, when it is expressed early on, it now says it will mature through the B lineage. We have the Pro B cell expressing CD20. The first thing is D to J rearrangment. This requires RAG. REMEMBER those! They're very important./ The enzyme terminal transferase is needed. Everything is not class switched. The pre B cell, V has rearranged to D. It can now encode a functional variable region. It's associated with an surrogate L chain because L has not rearrangeed yet. A functional H chain which nowgoes to the surface. The next thing that takes palce is l chain rerarragment. RAG is still present. IgM is on the surface. The next event is expression of IgM and IgD at the same time. That makres the cell as a mature B cell. Remember that both those antibodies recognize the same antigen. This mature B cell now has ready to exit the bone marrow and go to the peripheral lymphoid tissue. They become antigen dependent. It will become activated, it can secrete a little bit of immuno globulin. It can undeerg go class switching and somatic mutation. If the proper signals are given, it can furthur differentiatie into a plasma cell. They are basically a antibody secreteing cell. No more changes take place. D to J rearrangnment, then V to D and expression with surrogate l chain. Then l chain rearrangmentment and iGM on the immature B cell. When it matures so that i t makes both IgM and IgD, it can go out. If it encounters antigen, it gets activated and becomes plasma cells. B220 is on all b cells./ KNOW B220 that it is a characteristic marker of the B cells. There's now rearragnment is the L chain lous. Ypou do't undergo somati mutation until you go into the periphery. This mature B cell will now leave the bone marrow. Th eB cells needs to meet up. There's secreted form and a transmembrane form. Allelic exclusion ensures that you only make one binding specificity. If you don't a V to J rearrangment on both alleles, the cell dies. ...
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- Fall '06