Blood: Lymphocytes and Monocytes

00:00 Let us move on to the agranulocytes, the lymphocytes.

00:06 I am going to talk about these cells later on when we talk about the immune system.

00:13 They are very common in blood. They occupy about 28 to 30 percent. So when you look at blood smears, you can see a number of this different sorts of lymphocytes. When they are traveling in blood, they are immunocompetent. But they are waiting to be attracted and identify antigens in tissues. So they actually continually leave the blood through lymph nodes that I will talk about in a later lecture. They circulate through the lymph node, being immunocompetent that recognized the antigens. They have been designed to recognize and take care of an antigen or at least initiate that immune response against an antigen. If they don't come across the antigens, they can leave the lymph node and then they can get into the blood system and recirculate through the body again looking for these foreign antigens.

01:14 They are on surveillance duty, but they do not do their job in blood. They only use blood to transport themselves to different locations in the body. There are very small lymphocytes.

01:27 There are very large lymphocytes. They range from roughly 6 to 8 microns to about 18 microns in size. A granulocyte that is very important to understand is the monocyte. It is a huge cell. It is about 18 to 20 microns in diameter when you see it in a blood smear. Now again, they are traveling through blood and they are not doing anything within the blood itself.

01:57 It is only when they moved into connective tissue that they do their job. They transform into being a macrophage and we will learn about these macrophages when we look at all the tissues of the body and all the organs of the body in more detail. They have a very characteristic indented nucleus, a bean-shaped nucleus and that enables them to be identified, apart from their massive sizes well relative to the other blood cells. Now again reflect on what I've said earlier about the orientation of cells in the blood smear. You are looking at a whole cell here. If that monocyte that was rotated in another direction, you may not see that in indented nucleus, so it may make it difficult to say that some monocyte could, in fact, be a mast cells travel through the blood and they look like monocytes.

03:04 It is only when they move into the connective tissue, those mass cells then accumulate the granules that they use in their role as being vasoactive agents and mediators.