Histology Structures Class Notes

‭Histology structures‬ ‭❖‬ ‭We oppose two structures.‬ ‭ -‬ 1 ‭A lymphoepithelial organ,‬‭it is the thymus in which‬‭the epithelial cells go constitute a‬ ‭network of meshes between which we will have the lymphoid cells.‬ ‭ -‬ 2 ‭Lymphohistiocytic organs‬‭:where the lymphoid cells‬‭are dispersed on a framework of‬ ‭macrophage connective cells with reticulin fibers;this is the case of the spleen, lymph nodes.‬ ‭At the level of lymphoid tissue we have a contingent of lymphoid cells which is fixed at the level of‬ ‭the organs and a contingent of circulatory cells.lantes which will transport the information. On a‬ ‭functional level, we distinguish:‬ ‭➔‬ ‭the central lymphoid organs in‬‭which we put the thymus,‬‭the hematopoietic‬ ‭bone marrow‬ ‭➔‬ ‭peripheral lymphoid organs where‬‭there are elements‬‭that will be differentiated (LB and‬ ‭LT), it is the spleen, the lymph nodes, the lymphoid formations associated with the mucous‬ ‭membranes which are called MALT (mucosa associated lymphoid tissue).‬ ‭In the peripheral lymphoid organs we distinguish two zones: the thymus-‬ ‭independent zones where we find B lymphocytes and the thymus-dependent zones where we‬ ‭mainly find T lymphocytes.‬ ‭➔‬ ‭plan :‬‭We see the afferent lymphatic channels, we generally‬‭distinguish two zones: in the‬ ‭cortical zone we can see structures with an ovoid appearance, these are lymphoid follicles which are‬ ‭independent thymic zones where we find B lymphocytes. distinguishes primary lymphoid follicles‬ ‭where there has been no antigenic stimulation and At this point this lymphoid follicle is simply‬ ‭made up of small lymphocytes which are very tight against each other, therefore a very dense,‬ ‭uniform cluster.‬ ‭❖‬ ‭If there is antigenic stimulation, we can distinguish 2 large zones:‬ ‭➔‬ ‭a peripheral zone which‬‭is dense where we will also‬‭find small lymphocytes, This is the‬ ‭mantle area. The small lymphocytes present in this coat are either naive B lymphocytes or memory‬ ‭LBs.‬ ‭➔‬ ‭a central area called the germinal‬‭center in which‬‭we distinguish 2 zones: a dark zone and‬ ‭a lighter zone. In the dark zone we find the centroblasts so following antigenic stimulation there‬ ‭was a proliferation of LB which initially is polyclonal / not very well oriented by relation to the‬ ‭antigenic stimulus and then it is a proliferation which becomes more monoclonal/more oriented in‬ ‭relation to the antigen against which the organism must defend itself therefore these centroblasts‬ ‭become centrocytes, or we have centrocytes interesting to fight against the antigen and at this‬ ‭moment we deliver a signal to them survival or they are not interesting: no survival signal and the‬ ‭centrocytes degenerate and are destroyed by phagocytosis.‬ ‭❖‬ A ‭ ddressing lymphocyte migration‬ ‭The immune system performs a defense function in relation to infectious agents, in relation to‬ ‭degenerating cells, to tumor cells, a surveillance function and a combat mission. For the‬ ‭surveillance function we need patrol cells, sentinel cells and these are represented by dendritic‬ ‭cells and certain T lymphocytes. These cells will provide the information and at that time the‬ ‭system immune system and alert liver will be able to implementace the fighters to eliminate the‬ ‭intruder, the combatants being the cytotoxic LT and the LB.‬ ‭Langhéran cells leave the epidermis because there is a drop in the expression of cadherins, a‬ ‭local inflammatory context and it is TNF alpha which will promote the modification of the‬ e‭ xpression of surface molecules and that is the migration of the cells of Langhérans. We will‬ ‭have at the level of the afferent lymphatic channels the cells dendritic cells which will become‬ ‭veiled cells and when they penetrate the parenchyma ganglion these cells become the‬ ‭interdigitate cells and come into contact with the T lymphocytes which will then allow the‬ ‭expansion of the humoral reaction which will be developed in secondary lymphoid follicles.‬