Lymphopoiesis: The Development of Lymphocytes
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Lymphocytes are the smallest and the second most common type of white blood cell. They are known as the cells of immunity. Lymphocytes have large round/oval nuclei that occupies most of the cell with little cytoplasm. The nucleus will stain dark purple or blue when it is exposed to a certain stain called Wright’s stain (please see figure 1 ).
As we already know, HSC can develop into common lymphoid progenitor (CLP) or common myeloid progenitor (CMP). CLP then will generate T and B lymphocytes while CMP generates myeloid elements. So, lymphocytes originate from CLP in the bone marrow.
One should note that during postnatal life, the bone marrow and thymus are the primary lymphoid organs. The secondary lymphoid organs in which specific immune response take place are the lymph nodes, spleen and lymphoid tissue.
Their development is altered by cytokines. Interleukin-7 interacts with stem cell factor to begin the process. In order for the cells to function, they will express on their surface immunoglobulin (Ig).
First there will be an expression of IgM, then IgD and finally IgG or IgA. The termination of B cell will take place in the peripheral lymph organs such as the spleen and lymph nodes. In these organs B cells change into plasma cells and memory cells.
T- Lymphocyte development begins with CLP cells that migrate to the thymus where they will differentiate into mature T cells. It is associated with the movement of the cells through the cortex and medulla of the thymus. Maturing begins in the cortex, and as the cells develop more they move towards the medulla.
During the developmental stages of T cells, the cells contain specific surface proteins. As the cell progresses through maturity, they are identified by antibodies that recognize the surface proteins. In other words, there are cell surface markers which are used by antibodies to identify the T cell.
T cell eventually differentiates into two types of T lymphocytes: cytotoxic T-cell and helper T-cell. Cytotoxic T-cell is important because they produce a substance called lymphokines which help B cells destroy foreign substances. Cytotoxic T-cells also have CD8 antigen.
Like B-cells, T-cells depend on interleukin-7 and other interleukins. One should note that most of T-cell development occurs in the thymus; however the final steps in which cytotoxic and helper T-cell are produced occur in the peripheral blood.