Red blood cells erythrocytes : These transport oxygen and hemoglobin throughout the body. White blood cells leukocytes : These support the immune system. There are several different types of white blood cells:. Current research endorses a theory of hematopoiesis called the monophyletic theory. This theory says that one type of stem cell produces all types of blood cells. Sometimes called primitive hematopoiesis , hematopoiesis in the embryo produces only red blood cells that can provide developing organs with oxygen.
At this stage in development, the yolk sac, which nourishes the embryo until the placenta is fully developed, controls hematopoiesis. As the embryo continues to develop, the hematopoiesis process moves to the liver, the spleen, and bone marrow , and begins producing other types of blood cells.
In adults, hematopoiesis of red blood cells and platelets occurs primarily in the bone marrow. In infants and children, it may also continue in the spleen and liver.
The lymph system, particularly the spleen, lymph nodes, and thymus, produces a type of white blood cell called lymphocytes. Tissue in the liver, spleen, lymph nodes and some other organs produce another type of white blood cells, called monocytes. The body continually manufactures new blood cells to replace old ones. White blood cells have the shortest life span, sometimes surviving just a few hours to a few days, while red blood cells can last up to days or so.
The process of hematopoiesis begins with an unspecialized stem cell. This stem cell multiplies, and some of these new cells transform into precursor cells. These are cells that are destined to become a particular type of blood cell but are not yet fully developed.
However, these immature cells soon divide and mature into blood components, such as red and white blood cells, or platelets. Although researchers understand the basics of hematopoiesis, there is an-ongoing scientific debate about how the stem cells that play a role in hematopoiesis are formed. Each type of blood cell follows a slightly different path of hematopoiesis. All begin as stem cells called multipotent hematopoietic stem cells HSC.
From there, hematopoiesis follows two distinct pathways. Trilineage hematopoiesis refers to the production of three types of blood cells: platelets, red blood cells, and white blood cells. The B cells are responsible for production of globular proteins known as immunoglobulins that are directed against antigens.
B cell markers include the CD19 and CD20 antigens. The immunoglobulins are, therefore, antibodies. The cells specialized to produce immunoglobulins are plasma cells. The plasma cells have an eccentrically placed nucleus with radially-arranged chromatin, a prominent Golgi apparatus next to the nucleus the perinuclear "hof" , and abundant cytoplasmic endoplasmic reticulum for synthesizing immunoglobulin. Plasma cells specialize in terms of antibody production.
When the immune system responds to an infection, there are typically antibody responses to a variety of antigens, so that there are different types of immunoglobulin produced with specificities for the antigens.
Such a response is called "polyclonal" because several clones of plasma cells, each producing a specific antibody, are stimulated to produce different immunoglobulins mostly IgG, but the earliest production of antibody is of the IgM type. A neoplastic transformation of B cells that results in a proliferation of plasma cells causes a "monoclonal" production of immunoglobulin, because the neoplasm called a myeloma is a proliferation derived from a single clone.
The use of immunizations e. An encounter with the real infectious agent at a later date will result in quick production of antibodies against the infectious agent, because there are circulating antibodies as well as plasma cells in "storage" waiting to respond to the challenge. The T cells are primarily involved in cell-mediated immune responses.
They are identified by the presence of the CD3 antigen. In fetal life and childhood, the T cells arose in the thymus and then populated other lymphoid tissues. The major T cells are the "helper" cells that mark with the CD4 antigen and the "suppressor" cells that mark with the CD8 antigen. The recognition and stimulation of T cells is largely dependent upon recognition of major histocompatibility complex MHC antigens, also known as HLA antigens.
Many cells of the human body express these antigens. The T cells are often aided by macrophages , which phagocytize infectious agents, process the antigens, and present them to the T cells and to B cells. Antibodies coating an invading micro-organism can lead to lysis by specialized lymphocytes called "natural killer" or NK cells. The granules contain mediators of the inflammatory process such as histamine. Dendritic reticulum cells serve to assist in storing and transferring antigens to lymphoid cells which will respond to the antigens.
The dendritic cells located in skin and mucous membranes are known as Langerhans cells. Within lymph nodes, they are known as follicular dendritic cells. These dendritic cells are named because of the long cell processes which catch antigens and "warehouse" them. The lymphatic channels of the body drain into groups of lymph nodes that are strategically placed to filter the lymph draining body regions and to provide immune surveillance based upon the antigen content of the lymph.
The afferent lymphatic channels drain into the periphery of a lymph node in a region under the connective tissue capsule known as the subcapsular sinus.
In the periphery of a lymph node is the paracortical region where lymphoid follicles are located. A follicle is a loosely arranged structure with an outer mantle of small T lymphocytes and a germinal center composed of B lymphocytes, follicular dendritic cells, and macrophages.
The interfollicular zones between the follicles are populated mainly by T cells. From the periphery of the node, connective tissue trabeculae extend toward the hilum of the node. Sinuses drain toward the hilum and contain mainly macrophages. The medullary cords located near the hilum of the node contain mainly plasma cells and small lymphocytes. From the hilum, the efferent lymphatic channels egress. In the lamina propria and submucosal regions of the gastrointestinal tract from the tongue to the colon are collections of lymphoid tissue.
In some areas, the lymphoid tissue is more prominent:. Pharyngeal Tonsil: these are the structures commonly called "tonsils" and comprise tissues functionally equivalent to lymph nodes. Peyer's Patches: these are ovoid areas from 0. The spleen has a connective tissue capsule and has trabeculae of connective tissue running through the parenchyma. Arterial blood reaches the spleen through the splenic artery and drains out the splenic vein. The splenic parenchyma consists of red pulp and white pulp.
The smaller arterial channels eventually become the central arterioles that run through the center of the nodules of white pulp, so named because this is where the lymphocytes are concentrated. From the arterioles, blood can be shunted directly to red pulp veins and out trabecular veins to the splenic vein. However, the filtering function of the spleen is accomplished by shunting some of the blood into the splenic cords. For instance, sometimes the bone marrow is stimulated in advance of a planned cancer therapy when profound suppression of the blood-forming cells in the marrow is expected.
Like any cell, the HSCs can undergo a mutation that leads to dysfunctional or malignant cells being produced rather than healthy cells. Depending on what stage of differentiation the cell is in when it makes this transformation, it gives rise to different types of disorders: myeloproliferative disorders, leukemias, lymphomas, and myelomas.
If the predominant cells in leukemia or lymphoma are more mature types, this indicates the cancerous transformation happened to a more mature cell or a cell that was closer to the final adult stage.
In lymphoma, there can be different lymphomas that reflect all different stages of lymphocyte development, including the developmental paths for B-cells and T-cells; thus, there are B-cell lymphomas, T-cell lymphomas, and even natural killer T-cell lymphomas. The treatment of leukemia, lymphoma and other blood cancers may involve a transplant of hematopoietic stem cells.
These can be your own cells, harvested from your bone marrow autologous , or from a donor allogeneic. Techniques used to obtain healthy blood-forming cells from the donor vary, but the transplant itself is a simple transfusion as the hematopoietic stem cells migrate from the blood into the bone marrow.
This is the term used for blood cell production that occurs outside of the bone marrow. It can be seen in chronic anemia, with the production of blood cells in the liver, spleen and sometimes in the lymph nodes.
In other scenarios, there may be malignant hematopoietic cells located in areas outside of the bone marrow. Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Kim CH. Homeostatic and pathogenic extramedullary hematopoiesis. J Blood Med. Microbiol Spectr. Your Privacy Rights. To change or withdraw your consent choices for VerywellHealth. At any time, you can update your settings through the "EU Privacy" link at the bottom of any page.
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