Blood is a liquid connective tissue performing many functions. Blood is formed out of 2 components, a solid component or the cellular component and the liquid plasma. The cellular components of the blood are WBCs, RBCs and platelets. WBCs is also known as the leukocytes help in natural defence mechanism that fights against foreign organisms and particles entering through various routes. WBCs differ from RBCs in many respects. WBCs are divided into 2 major categories: granulocytes and agranulocytes. In the normal blood, leukocyte count is approximately ranging between 5000 to 10,000 cells per cubic millimetre, out of which 60% to 70% are granulocytes and the remaining 30% to 40% are lymphocytes. However, they rapidly rise during infections and illnesses.
Granulocytes are named so because of their granular cytoplasm. Based on the staining properties of the granules, there are 3 subtypes of granulocytes; eosinophils, basophils and the neutrophils.Eosinophils have bright-red granules in their cytoplasm, whereas basophils are shown in deep blue stained granules. Neutrophils are also called polymorphonuclear neutrophils (PMNs, or polys). In a mature neutrophil, there are multiple lobes connected by thin filaments of nuclear material. However, the less mature granulocyte contains a single-lobed and elongated nucleus known as band cell. Band cells assist in fighting infections by rapidly multiplying themselves during the state of emergency. Granulocytes are produced from the myeloid stem cells. In the initial stage, they look like immature cells ( blast cells) but they gradually differentiate to form mature neutrophils through a process called myelopoiesis. As the blast cell is getting matured, the cytoplasm of the cells turns violet from its initial blue colour. Simultaneously, the granules begin to form with the cytoplasm. The process of maturation and differentiation of granulocytes takes hardly 10 days. Neutrophils are ready to participate in the phagocytosis process once they are released into the circulation from the bone marrow. Phagocytosis is the process of ingestion and digestion of bacteria and the foreign particles.
There are 2 types of agranulocytes, monocytes, and lymphocytes. As the name suggests, monocytes have a single-lobed nucleus. They do not contain granular cytoplasm - hence the term agranulocytes. In a healthy and normal adult, monocytes account for approximately 5% of the total WBCs. Monocytes are the largest WBCs produced by the bone marrow. They get transformed into macrophages when the body is attacked by infectious agents. WBCs are more active in the organs; spleen, liver, peritoneum, and the alveoli of the lungs because these organs are sensitive and react quickly to infecting agents.
Lymphocytes are the smallest WBCs produced by marrow released by the lymphoid stem cells. There are 2 types of lymphocytes; T- lymphocytes and B-lymphocytes.T-lymphocytes are produced in the cortex of the thymus gland whereas B-lymphocytes are produced from bone marrow.
The primary function of lymphocytes is to produce substances that aid in attacking foreign material. They take part in the phagocytosis ( especially neutrophils ). During the times of crisis, neutrophils reach the site of infection within 1 hour after the onset of an inflammatory response. Neutrophil invasion is followed by the entry of many monocytes to defend against the inflammatory agents.
WBCs produce macrophages; effective against fungi and viral agents. Macrophages also help in self-digestion of senescent (aged cells) blood cells.
T-lymphocytes assist in killing the foreign cells directly by releasing lymphokines( substances that enhance the activity of phagocytic cells).
T-lymphocytes also assist in delaying allergic reactions, rejection of foreign tissues (eg, transplanted organs), and destruction of the tumour cells through cellular immunity.
B –lymphocytes are capable of differentiating into plasma cells. Plasma cells, in turn, produce immunoglobulin (Ig), or antibodies, -protein molecules that destroy the foreign material by humoral immunity.
Eosinophils and basophils help during hypersensitivity reactions.
Production of WBCs
Formation of WBCs (Leukocyte Hematopoiesis)
The technical term used for the production of blood is the hematopoiesis. Leukocyte Hematopoiesis is the formation of WBCs from their stem cells called hematopoietic stem cells (HSCs). HSCs are found in the bone marrow with their unique ability to develop mature white blood cells. The primary HSCs undergoes a systematic differentiation and cell division to produce a massive number of leucocytes. Processes. There are 3 different lineages of WBCs. The lymphocyte lineage is the derivative of the lymphoid progenitor cells assist in the formation of primary lymphoblasts before they are differentiated into T cells, B cells, and NK cells. On the other hand, Myelocytes are proliferated from the common myeloid progenitor cells. These cells gets differentiated into the erythropoietic and megakaryocytic progenitors. This diverse group undergoes further differentiation to form the granulocytes and monocytes. The monocytes further differentiate to form the macrophages or dendritic cells upon reaching certain tissues.
Sites of Hematopoiesis
During the early embryonic phase, the blood production occurs in the yolk-sac, however, the mother`s placenta supplies the maximum requirement of the growing embryo. As development progresses, the formation of blood is shared by the spleen, liver, and some lymph nodes. In the young children and adults, hematopoiesis occurs in the marrow located in the hallow space of the long bones such as the femur and tibia. In addition, the adult`s pelvis, cranium, sternum and vertebral bones play an important role in the production of WBCs. Extramedullary hematopoiesis is another term used to indicate the production of blood away from the medullary spaces of bones. Some of the Extramedullary spaces participating in the production of blood are, the liver, thymus, and spleen.