In this video, we're going to continue to talk about adaptive immunity by introducing and differentiating primary versus secondary lymphoid organs. And so first we need to recall from some of our previous lesson videos that T and B lymphocytes, or T and B cells, are the primary cells involved in adaptive immunity. Where T cells are important for cell-mediated immunity, and B cells are important for humoral immunity. Now, the primary lymphoid organs are the specific organs within our bodies where immature T and B lymphocytes develop into their naive forms. And these naive T and B lymphocytes are really just inactive T and B lymphocytes. And so the naive T and B lymphocytes are fully developed, but they have not yet encountered the specific antigen that they respond to. And so they remain in their inactive forms when they are naive. However, whenever these naive T and B lymphocytes are introduced to their specific antigen, the naive T and B lymphocytes can become activated, and the activated T and B lymphocytes can go on to generate immune responses, which we'll get to talk a lot more about this process as we move forward in our course. Now these primary lymphoid organs are where the immature T and B lymphocytes develop into naive forms. And the primary lymphoid organs include the thymus, which is where the T cells develop. And so you can see the T in thymus can help remind you that this is where T cells develop. And, the bone marrow is also a primary lymphoid organ, and the bone marrow is specifically where B cells develop. And so the B in bone marrow can remind you of the B in B cells. Now although T cells develop in the thymus and B cells develop in the bone marrow, it is important to note that both the T and B lymphocytes are actually initially produced in the bone marrow together. However, the T cells will go on to migrate from the bone marrow to the thymus where they will fully develop, and the B cells remain in the bone marrow to fully develop.
Now the secondary lymphoid organs, on the other hand, are organs where the naive or inactive T and B lymphocytes can become activated. And the activated T and B lymphocytes can then begin their immune functions, which we'll get to talk about their immune functions in more detail as we move forward in our course. Now in other words, we can say that these secondary lymphoid organs are specific sites within our bodies where foreign molecules or foreign antigens are brought into contact with large populations of naive lymphocytes, activating those naive lymphocytes so that they can begin their immune functions. Now the secondary lymphoid organs include the lymph nodes, the spleen, and tonsils. Notice that the primary lymphoid organs are going to have the number 1 next to them and the secondary lymphoid organs are going to have the number 2 next to them in this image. And so, once again recall that the organs include the thymus, which is where the T cells are going to fully develop. Notice that the thymus is the specific organ that is right above the heart as you see right here. The primary lymphoid organs also include the bone marrow. And the bone marrow again is where the B cells are going to fully develop. But once again, don't forget that both T and B cells will be initially produced in the bone marrow, but then the T cells migrate to the thymus to fully develop, whereas the B cells remain in the bone marrow to fully develop.
The secondary lymphoid organs once again include the tonsils, spleen, as well as our lymph nodes. And so this here concludes our brief introduction to primary and secondary lymphoid organs, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.