In this video, we're going to begin our lesson on secondary lymphoid organs with an introduction to lymph nodes. Now lymph nodes are small bean-shaped structures, which is why we're showing you some images of beans down below so that you can compare them to the typical structure of a lymph node that we have right here. And notice that both of them have an indentation that you can see right here. This indentation in the lymph node is called the hilum. Now, these lymph nodes are found all along lymphatic vessels throughout practically the entire body, from your hands and your feet up through your arms and legs, into your torso region, and even up into your head. However, larger and more prominent lymph nodes are strategically located in regions of the body where lymphatic vessels tend to merge to form lymphatic trunks. And those are going to be the regions that we have indicated here in this image on the right-hand side, which are the cervical region or the cervical lymph nodes, found here in the neck region, the axillary lymph nodes, or the axillary region, which is the armpit or under the arm, and the inguinal lymph nodes or the inguinal region, which is this region where your thighs meet the abdomen.
Now, the primary function of these lymph nodes is to filter the lymph of harmful pathogens and foreign substances, and these lymph nodes are incredibly good at filtering the lymph. In fact, it's estimated that about 99% of all the pathogens and foreign substances that arrive at a lymph node are trapped within the lymph node and filtered from the lymph before the lymph is returned back to the bloodstream. So that's pretty remarkable, and it goes to show how important these lymph nodes are to our immunity. Now the amount, size, and shape of lymph nodes actually varies drastically from individual to individual. In terms of the amount, we can't quite put an exact number on it. However, typically, there are hundreds of lymph nodes in a single person. In fact, it's estimated that there can be anywhere from about 400 up to about 700 lymph nodes in a single person, which makes lymph nodes the most numerous lymphoid organ by far. Again, the size can also vary anywhere from about just a few millimeters in diameter up to over an inch in diameter. And although the shape is typically bean-shaped, the shape can also vary. But not only does the amount, size, and shape of lymph nodes vary from individual to individual, but it is also dynamic within each of us. The amount, size, and shape of lymph nodes can change within each of us depending on the specific situation and specific factors such as, for example, infection, inflammation, and immune activity. It's well known that when you do get sick, your lymph nodes tend to swell up and enlarge, and you can often feel them in your neck region when you are sick.
Because lymph nodes are secondary lymphoid organs, it's no surprise that each lymph node contains lots of immunocompetent lymphocytes, such as T and B lymphocytes, or T and B cells. And although the primary function of lymph nodes is to filter the lymph and trap pathogens, lymph nodes are also critical for immune system activation, essentially stimulating the immune system to generate a response. This is what makes these lymph nodes secondary lymphoid organs. Now, later in our course, we're going to talk a lot more details about immunity and a lot of these specific cells, and their roles. However, recall from previous lesson videos that dendritic cells are cells that are able to capture antigens in distant tissues, and they can actually transport those antigens to nearby lymph nodes and present those antigens to lymphocytes to activate the lymphocytes. Those activated lymphocytes could either carry out an immune response right then and there in that lymph node, or the activated lymphocytes may actually enter into the bloodstream and migrate to that distant site of infection to generate the immune response in that distant site of infection. Again, these are ideas that we'll get to talk more about later in our course when we're talking about immunity. But this here concludes our brief introduction to lymph nodes, and as we move forward, we'll get to talk more about the structure of the lymph node. So I'll see you all in our next video.