in this basic concepts and immunology series we're going to learn about the lymphatic system we will learn about the different parts of the lymphatic system that include the primary and secondary lymphoid organs and tissues we will look at the structure of the lymph node and the spleen as well as the mucosa-associated lymphoid tissues the lymphatic system is a system in our body whose job is to transport a fluid called as lymph it forms a network similar to the circulatory system so in the figure we can look at the circulatory system and compare it with the lymphatic system and the way we have a network of blood vessels in the circulatory system to transport blood in the lymphatic system we will have lymphatic vessels that aid in the transportation of lymph now lymph is a fluid that is generated when we have fluid in the blood leak from the blood vessels into tissues what should be noted is that lymph is transported in the lymphatic vessels in only one direction so it is a unidirectional transport the lymphatic system plays a very important role in immune defense because it is in the lymphatic system where the lymphocytes are able to interact with antigens so lymphocytes are cells of the adaptive immune response and they get activated when they're able to sample antigens and so all this process happens in the lymphatic system let us look at how lymph is formed so when we look at this figure we can see a network of blood vessels and these are capillaries that arise from arteries and veins these capillaries are able to provide nutrition like oxygen and nutrients to tissue cells that are present now the fluid present in these capillaries can leak from the capillary walls and they go into the spaces that is there between the cells and tissues this fluid is called as interstitial fluid this interstitial fluid is very important because it provides the cells and tissues with oxygens and nutrients additionally it helps removing any waste substances that have been generated by the cells that are present in that tissue now the interstitial fluid that is leaking out of these capillaries has to be taken up and transported elsewhere or else you're going to end up with fluid accumulation this process is done by the lymph capillaries so the interstitial fluid that's going to have the way substances generated from the cells and can also have immune cells that leaked out from the capillaries is taken up by the lymph capillaries and when that happens that interstitial fluid is now called as lymph so this lymph or tissue fluid that enters into the lymph capillaries gets transported in one direction so the lymph capillaries then feed into lymphatic vessels which then continue to transport the lymph so lymph is basically interstitial fluid that is in a lymphatic capillary or lymphatic vessel let us see how the interstitial fluid is able to enter the lymphatic vessels now when you look at the lymphatic vessels which include the lymph capillaries they are made up of cells called as the lymphatic endothelial cells the way these lymphatic endothelial cells are arranged so if you focus on the figure they have these overlapping junctions when there is accumulation of interstitial fluid these junctions respond to that increase in pressure in the tissues and that results in these junctions to open as shown in those black circles in the figure below since the junctions have opened up it allows the interstitial fluid to enter into the lymphatic vessel which now results a decrease in the pressure that decrease in pressure is now going to let those overlapping junctions close again now as the interstitial fluid that has entered into this lymphatic vessel it's now called as lymph and the lymph moves along and it usually has immune cells also associated with it and will enter what's called as the afferent lymphatic vessel and it moves in only one direction there are valves present which are shown in the figure and these valves allow the unidirectional transport of lymph in the afferent lymphatic vessel to have an overview on how the lymphatic system works it begins with the leaking of a fluid from the blood capillaries which is called as the interstitial fluid and this fluid leaks into the spaces that is there in tissues this interstitial fluid is then going to enter the lymphatic vessels this interstitial fluid has waste products generated by cells but it also has immune cells and if there was any pathogen that was there it will also carry the pathogen and the pathogen products with it once the interstitial fluid enters the lymphatic vessel it's called as lymph and the lymphatic vessel will then transport the lymph from the tissue to a nearest structure that's called as a lymph node we're going to learn about lymph node in a bit but once the lymph reaches the lymph node then the lymph node will sample the lymph and then the lymph will be transported to another lymph node so this way it gets transported from one lymph node to another lymph node until the lymph ultimately reaches the thoracic duct once it reaches the thoracic duct it then empties into blood and so it is returned to blood where it is now part of blood again and it can be circulated back all throughout the body so this is how the lymphatic system works the different parts of the lymphatic system include the primary or central lymphoid organs and the secondary lymphoid organs or tissues the primary or central lymphoid organs are the places where the lymphocytes are made so these include two regions which are the bone marrow and the thymus the thymus is the place where we have the t lymphocytes mature and hence that is why it is also considered as a primary or central lymphoid organ the secondary or peripheral lymphoid organs includes places where adaptive immune responses are initiated they include organs as well as tissues so the parts that come under the secondary or peripheral lymphoid organs or tissues are the lymph nodes and there are many lymph nodes throughout the body the spleen which is an organ that is right near the stomach and the mucosal associated lymphoid tissues that are present in different regions of the body we're going to first look at the lymph nodes lymph nodes are being shaped structures that have immune cells like macrophages dendritic cells and lymphocytes now lymph can enter the lymph nodes through the afferent lymphatic vessels and once they enter into the lymph node it's sampled by all the different immune cells and the lymph will finally leave the lymph node through the efferent lymphatic vessel you also have blood supply to the lymph nodes so we do have arteries and veins and immune cells are that are in blood are able to squeeze out of these arteries and veins and gain access to lymph in the lymph node that way as well when we look at the structure of the lymph node the lymph node can be divided into three main regions the outer region is called as the cortex well the middle region is called as the paracortex and the innermost region is called as the medulla now the b lymphocytes or b cells are localized in the cortex which is the outer region in the lymph node in this cortex we can see the presence of lymphoid follicles and that is specifically where the b cells are they're found in the lymphoid follicles in the cortex now t lymphocytes on the other hand are abundantly present in the paracortical areas so this paracortex region where the t cells are is also called as the deep cortex and another name is the t cell zone now upon activation by exposure to antigens b and t lymphocytes migrate to a region or the periphery of where the cortex meets the pericortex now upon activation b cells can undergo proliferation and they form these structures called as germinal centers now in the figure wherever you can see that yellow circle in the lymphoid follicle that is a germinal center so a germinal center is a region where b lymphocytes are undergoing proliferation the spleen is a secondary lymphoid organ that is found behind the stomach it plays important roles in destroying old red blood cells but can also aid in immune defense because it can collect antigens that are present in the blood and thus it helps in immune responses against blood-borne pathogens or pathogens that are found in blood what should be remembered about the spleen is that it does not have lymphatic vessels so lymph never comes to the spleen only blood clumps to the spleen so antigens and immune cells that enter the spleen always enter through blood vessels and the specific blood vessel that brings blood supply to the spleen is the splenic artery the spleen is made up of two parts which is the red pulp and the white pulp bulk of the spleen is made up of red pulp now the red pulp is the area where red blood cells are disposed so it has an abundance of red blood cells and it also has macrophages and some lymphocytes the macrophages are the ones that will actually destroy the red blood cells by engulfing them through phagocytosis the second region is the white pulp now the white pulp is a region that is rich in lymphocytes and is usually found surrounding the arterioles as we can observe in the figure between the red pulp and the white pulp the region is called as the marginal zone now the white pulp is made up of different regions so let's look at some of them one region is the periarteriolar lymphoid sheath or pals and this is the region which surrounds the arterial and is very rich in t cells we also have lymphoid follicles and in the lymphatic follicles we will see b cells abundantly now what should be noted is as we have seen in the past that certain b cells in the follicle could undergo activation and start proliferating and when that happens they form germinal centers in these follicles so in the picture below where we can see those light yellow regions right in the middle where you see the dark yellow circles those are the germinal centers where b cells are underground proliferation now these b cells are activated b cells the region between the red pulp and the white pulp as we saw before is the marginal zone we're now going to look at secondary lymphoid tissues secondary lymphoid tissues are abundantly found on mucosal surfaces mucosal surfaces are regions of the body that are more accessible to the outside environment like the lining of your gastrointestinal tract or the lining of your respiratory tract or the urogenital tract the mucosal surfaces are very accessible to microorganisms due to their proximity to the external environment hence they have to be protected very well and that is where the secondary lymphoid tissues come into play which are called as mucosa-associated lymphoid tissues or malt now depending on the location the mold can have more specific names for example in the gut the mold is also called as the gut associated lymphoid tissues on the other hand when we look at the bronchial regions we have bronchus-associated lymphoid tissues similarly we have nasal associated lymphoid tissues and finally in the skin we can find the skin-associated lymphoid tissues in this picture we are looking at one of the gut associated lymphoid tissues and a lot of times these lymphoid tissues are going to have lymphocytes like t-cells and b-cells but they also have dendritic cells now when we look at the picture the part that is in blue is the region that has a lot of t lymphocytes on the other hand you can see follicles where we can observe many b lymphocytes finally we do have dendritic cells that are constantly sampling the environment looking for antigens and displaying these antigens or looking for pathogens phagocytosing them and displaying the antigens with this we come to the end of the lymphatic system where we learned about primary and secondary lymphoid organs and tissues we also learned about the structure of the lymph node and the spleen and finally we looked at mucosa-associated lymphoid tissues