In this video, we're going to talk about lymphoid tissue. Now lymphoid tissue is actually made up of reticular connective tissue, which, recall from way back in our previous lesson videos, is a type of loose connective tissue that has reticular protein fibers in its extracellular matrix, which have a net-like or branched structure. Now lymphoid tissue is also going to be made up of the lymphoid cells that we talked about in our last lesson video. Now, the primary function of lymphoid tissue is to support the proliferation and activation of immune cells, particularly lymphocytes, such as B cells and T cells. Now lymphoid tissue can actually be found in 2 different arrangements and distributions. The arrangements refer to how densely packed the lymphoid tissue is, and the distributions refer to where the lymphoid tissue can be found in the body.
The first is going to be diffuse lymphoid tissue, which as its name implies with the term diffuse, is going to have a diffuse arrangement or a loose arrangement of lymphoid tissue. And in terms of its distribution, it's going to be widely distributed in the body. In fact, diffuse lymphoid tissue is closely associated with areolar connective tissue, which, recall from way back in our lesson videos, is another type of loose connective tissue that is very widely distributed and can be found underneath practically all epithelial tissue, including underneath our skin, all around our body, and underneath the mucous membranes that line our respiratory and digestive tracts. And so, if we take a look at our image down below over here on the left hand side, notice that we're zooming into regions on this person's body from their head all the way down to their feet, which goes to show how widely distributed this diffuse lymphoid tissue is. And from this micrograph here, you can see the loose arrangement of the lymphoid tissue where the protein fibers are very loosely arranged and you can see these white spaces in the background.
Now, the next type of arrangement and distribution are going to be the lymphoid follicles, which are also sometimes referred to as lymphoid nodules. Now, be careful not to confuse lymphoid follicles or lymphoid nodules with lymph nodes, which are a lymphoid organ, whereas lymphoid follicles and lymphoid nodules are tissues that are found within lymphoid organs. Now in terms of their arrangements, lymphoid follicles or lymphoid nodules are going to be very densely packed and consist of densely packed spheres of lymphoid tissue. And in terms of their distribution, they are not as widely distributed as diffuse lymphoid tissue. And really, lymphoid follicles or lymphoid nodules are only really found in lymphoid organs that we'll get to talk more about moving forward in our course. Now often, these lymphoid follicles or lymphoid nodules will have what are known as germinal centers, which are the central area of a lymphoid follicle or lymphoid nodule, where B cells or B lymphocytes actively proliferate and divide. And these lymphoid follicles and lymphoid nodules are actually pretty dynamic, and they can come and go as pathogens invade the tissues and as those pathogens are eliminated. However, lymphoid follicles and lymphoid nodules are a pretty consistent feature of some lymphoid organs, including lymph nodes. And so notice down below, we're showing you a lymph node structure, and we'll get to talk more about the lymph node structure as we move forward in our course. But what I'd like you to notice here is that we're highlighting one of the lymphoid follicles found within this lymph node, and so notice that the lymphoid follicle is going to be this densely packed sphere of lymphoid tissue that you can see highlighted right there. And again, often the lymphoid follicles are going to have a lighter stained germinal center where B cells are going to actively proliferate and divide.
Now it turns out that practically all lymphoid organs that we're going to talk about moving forward in our course have these lymphoid follicles or lymphoid nodules except for the thymus, which as we'll learn moving forward in our course, is an exception because they lack the lymphoid follicles and nodules, and they lack B cells. And, again, as we'll learn moving forward in our course, the thymus is almost exclusively specific to the development of T cells. And so, really, this concludes our lesson on lymphoid tissue, and we'll be able to apply these concepts and continue to learn more as we move forward in our course. So, I'll see you all in our next video.