In immunology, white blood cells can be classified as polymorphonuclear neutrophils, polymorphonuclear basophils, polymorphonuclear eosinophils, monocytes, lymphocytes, or plasma cells. Lymphopoiesis is the process of producing lymphocytes, such as B cells, T cells, and natural killer cells, in the bone marrow. In this process, progenitor cells in the bone marrow differentiate into lymphocytes. Lymphopoiesis is necessary for survival because mature lymphocytes are essential elements of the body’s lymphatic system.
The formal term for lymphopoiesis is lymphoid hematopoiesis, which basically means the production of blood cells called lymphocytes. Undifferentiated cells, called pluripotential hematopoietic stem cells, in the bone marrow can undergo a series of cell divisions and differentiations before committing to either the production of red blood cells, myelocytes, or lymphocytes. In lymphopoiesis, the pluripotential hematopoietic stem cell gives rise to the multipotent progenitor cell. This cell gives rise to the early lymphoid progenitor, which in turn gives rise to the common lymphoid progenitor (CLP). The common lymphoid progenitor can give rise to natural killer (NK) cells, dendritic cells, and prolymphocytes.
In the lymphopoiesis of T cells, the lymphocytes are first formed in the bone marrow and are then transported to the thymic cortex where they undergo maturation. The T cells in the thymus stay in an antigen-free environment for almost 1 week. Only 2 to 4% of the original population of T cells is able to survive in this environment.
Other T cells either undergo apoptosis or are eaten and destroyed by macrophages. The death of this large amount of T lymphocytes ensures that the surviving lymphocytes can recognize self-major histocompatibility complexes (MHCs). Recognition of this complex prevents the autoimmune destruction of the body’s own cells. T cells or thymocytes may differentiate into helper T (Th) cells, cytotoxic T (Tc) cells, memory T cells, and suppressor or regulatory T cells.
In the lymphopoiesis of B cells, B lymphocytes are initially formed in the bone marrow. When the bone marrow is impaired, the spleen could take over this function. The first studies on B cells were done on the bursa of Fabricus present in chickens, and this is why they are called B cells. After formation, B cells are then transported to lymph nodes and introduced to antigens.
Antigen recognition is an important function of B cells. Once a B cell recognizes an antigen, it becomes activated and differentiates into the plasma cell, an antibody-secreting cell. Antibodies bind the antigen and stimulate destructive mechanisms, such as the complement system and macrophage phagocytosis. The most common antibody secreted is immunoglobulin G (IgG). Other antibodies, such as immunoglobulin A (IgA), immunoglobulin E (IgE), and immunoglobulin M (IgM), may also be manufactured by mature B cells.