Anatomy and development of lymphocytes
Lymphocytes are with 6-12 μm very size-variable and are particularly noticeable by the large dark cell nucleus, which fills almost the entire cell. The rest of the cell can be recognized as a thin cytoplasmic fringe, which contains only a few mitochondria for energy production and ribosomes for the production of proteins. It is assumed that the larger forms of lymphocytes, which also have a lighter (=euchromatic) cell nucleus, were activated by bacterial or viral attack.
Smaller inactive lymphocytes, which are also called naive, are much more common in healthy people and are about the same size as the red blood cells (erythrocytes). Lymphocytes are formed via the intermediate stage of lymphoblasts from hematopoietic stem cells (hematopoiesis = blood formation), which in adults are mostly found in the bone marrow. Here the progenitor cells (progenitors) of the lymphocytes differ from those of the other (myeloid) cells at a very early stage in that some of them continue to mature in the thymus (also called sweetbreads).
These are later called T lymphocytes (“T” as in thymus). The purpose of maturation in the thymus is to sort out all those T-cells that react to the body’s own structures or are otherwise restricted in their function (positive and negative selection). The B-lymphocytes and NK-cells (natural killer cells) on the other hand complete their maturation like the other blood cells in the bone marrow (“B” = bone marrow or historically Bursa fabricii, an organ of birds).
After the B lymphocytes have left the bone marrow as mature, naive (=unspecialized) cells, they enter organs such as the spleen, tonsils or lymph nodes, where they can come into contact with antigens (structures foreign to the body). For this purpose, the cell carries certain antibodies on its surface, which serve as B-cell receptors.So-called dendritic cells, another type of immune cell that does not belong to the lymphocytes, present antigen fragments to the naive B-lymphocytes and activate them with the help of the T-helper cells. If a B cell is activated, it divides several times and transforms into a plasma cell (clonal selection).
The different lymphocyte types look very similar, but can be distinguished from each other under the microscope using special marking and staining methods (immunohistochemistry). The B-lymphocytes and NK-cells (natural killer cells), on the other hand, complete their maturation like the other blood cells in the bone marrow (“B” or historically Bursa fabricii, an organ of birds). After the B lymphocytes have left the bone marrow as mature, naive (=unspecialized) cells, they enter organs such as the spleen, tonsils or lymph nodes, where they can come into contact with antigens (structures foreign to the body).
For this purpose, the cell carries certain antibodies on its surface, which serve as B-cell receptors. So-called dendritic cells, another type of immune cell that does not belong to the lymphocytes, present antigen fragments to the naive B-lymphocytes and activate them with the help of the T-helper cells. If a B cell is activated, it divides several times and transforms into a plasma cell (clonal selection). The different lymphocyte types look very similar, but can be distinguished from each other under the microscope using special marking and staining methods (immunohistochemistry).
