Lymphoid System

©1996 University of Kansas Medical Center

Department of Anatomy and Cell Biology

Dont' lose perspective! The lymphoid system may be presented here in it's own lesson, but don't forget that lymphocytes are blood cells. They are, of course, leukocytes. Below is the same overview of hematopoietic tissues found in the blood chapter. It is worth repeating.

Advice from a second-year medical student...

Do yourself a big favor and get a good handle on blood. You'll be faced with blood time and time again in immunology and pathology. Many times, the only way you can know what type of process is going on is to know your blood. Different inflammations, for example, are differentiated by what type of white blood cell is present. You'll study that later.

For some reason, blood tends to intimidate med students (especially when it's their own), but don't let it! Start with a simple framework and then you can fill in the details.

You have two types of blood cells:
These are pretty straight forward. Look at the Erythroid Series.

Here's where the action is, but you'll see it's pretty straight forward as well. The leukocytes are part of our immune defense system.

There are three types of leukocytes:
    1. Granulocytes (These are also called 'Myeloid Cells.')
    2. Monocytes
    3. Lymphocytes

    Some sources combine the monocytes and lymphocytes into one category and just call them all 'mononuclear leukocytes.' (You probably are beginning to realize that the real challenge in learning this is just learning the vocabulary.)

Granulocytes inlcude the neutrophils, eosinophils, and basophils. (It shouldn't surprise you to learn that their cytoplasm is often filled with granules.) These are the work horses of acute inflammation (and other processes). Make sure you learn the neutrophil. In pathology and immunology, you'll call this same cell the PMN (polymorphonuclear leukocyte). Eosinophils are involved in allergic reactions and parasitic infections. Another cell to mention here (although it is NOT a granulocyte) is the Mast Cell. It's very similar to the basophil: both release histamine (and other mediators). Some think that the mast cell is derived from the basophil. Just remember that the basophil circulates and the mast cell is found in peripheral tissues. Other than that, they are quite similar.

Monocytes include the monoctye and the macrophage. The monocyte circulates in the blood until it receives the signal to extravasate into the peripheral tissue. Once in the tissue, it matures into the macrophage. It can also mature even further into other cells, but that is beyond the scope of this course. The monocyte/macrophage is the work horse of chronic inflammation.

Lymphoctyes are often overlooked when we consider blood, but they are white blood cells. Indeed, they originate in the bone marrow and are derived from the same stem cell as the rest of the erythrocytes and leukocytes. These are the T-cells and B-cells that direct the immune system and produce antibodies, respectively. They are the central cells in our cell-mediated and humoral (antibody) defense mechanisms. Also keep in mind that the B-cell can mature into the plasma cell.

If you remember nothing else... Know that as a rule of thumb, bacterial infections cause granulocytosis and viral infections cause lymphocytosis. There are exceptions to this, but this is a very basic (and important) concept. You'll learn more about why this is in immunology.

This should give you a good place to start. Spend some time learning the pathways from the original stem cell and realize that it gives rise to all these cells. It will be well worth your time to really learn this. You will be glad that you did (just ask any pathology student). Your goal here should be more than to just get by.

Good Luck!