| Bone Marrow |
This is where all cells of the immune system and the red blood cells are generated. In fact, all of them come from a common stem cell. This primitive stem cell differentiates into various clones that give rise to the major cell lines. |
| Thymus |
This organ is the "finishing school" for T lymphocytes that is active in fetal life and childhood. During fetal life, T lymphocytes proliferate and develop in the thymus. Enough T cells proliferate to be able to recognize millions of potential antigens. As a result only a fraction of the body's total lymphocyte population can react to any one specific antigen, but T cells circulate through the vascular system (lymphatics, arteries, veins, capillaries) and organs to give them opportunity to contact antigens. Such a specific T lymphocyte can generally find the antigen within a day. |
| Lymph Nodes |
Contain a variety of cells and function to gather lymph from peripheral tissues. The lymph drainage carries antigens that are processed and initiate immune responses. |
| Gut-Associated Lymphoid Tissue (GALT) |
The lymphocytes and lymphoid follicles and Peyer's patches that are present in the mucosa and submucosa of the gastrointestinal tract. The microfold cells (M-cells) of the intestinal epithelium help to process and display antigens to GALT. Collectively, GALT is the largest lymphoid organ of the body. |
| Spleen |
The spleen has a lymphoid population. The most prominent part of this population appears in the "white pulp" that consists of lymphocytes that cuff the arterioles. Additional lymphocytes as well as macrophages and granulocytes can be found in the red pulp. The spleen is a big blood filter. |
| Mononuclear Phagocyte System |
There are tissues that contain a population of cells that are permanent residents and which have a macrophage-like function. They participate in immune surveillance. Such cells include:
Kupffer cells lining hepatic sinusoids
Macrophages near bone marrow sinusoids
Macrophages in splenic sinusoids
Microglial cells of the central nervous system |
| Session 1 Index | The Major Organs of the Immune System | Cells of the Immune System | Major Histocompatibility Complex | Peripheral Blood Components | Sets of Laboratory Findings to Interpret |