White Blood Cells (WBCs) or leukocytes

Introduction

Blood is a liquid connective tissue help in 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 that help in natural defence mechanism to fight against the foreign organisms and the 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 roughly ranging between 5000 and 10,000 cells per cubic millimetre, out of which 60% to 70% are granulocytes and the remaining 30% to 40% are lymphocytes. Nevertheless, they rapidly increase during infections and illnesses.                 


Table of contents


Different types of blood cells

 

 



Granulocytes

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 holds a single-lobed and elongated nucleus known as band cell. Band cells aid in fighting against infections as they can by rapidly multiply themselves during the state of emergency. Granulocytes are produced from the myeloid stem cells in the bone marrow. 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 foreign particles.


Agranulocytes or mononuclear WBCs

There are 2 types of agranulocytes, monocytes, and lymphocytes. As the name implies, 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 converted 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 against 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.


 

Functions of WBCs

  1. The principal 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.

  2. WBCs produce macrophages; effective against fungi and viral agents. Macrophages also help in self-digestion of senescent (aged cells) blood cells.

  3. T-lymphocytes assist in killing the foreign cells directly by releasing lymphokines( substances that enhance the activity of phagocytic cells).

  4. T-lymphocytes further assist in blocking allergic reactions, rejection of foreign tissues (eg, transplanted organs), and destruction of the tumour cells through cellular immunity.

  5. 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.

  6. 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 haematopoiesis. Leukocyte Haematopoiesis is the formation of WBCs from their stem cells, haematopoietic stem cells (HSCs). HSCs are found in the bone marrow with their unparalleled ability to develop matured white blood cells. The primary HSCs undergoes a systematic differentiation and cell division to produce an extensive number of leucocytes.  There are 3 different lineages of WBCs. The lymphocyte lineage- the derivative of the lymphoid progenitor cells assist in the production 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. The progenitor cells are differentiated into erythropoietic and megakaryocytic progenitors. The diverse group encounters additional differentiation to form the granulocytes and monocytes. The monocytes further differentiate to form the macrophages or dendritic cells upon reaching our peripheral body tissues during infective crisis.

 

Sites of Haematopoiesis

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 the development progresses, the formation of blood is shared by the spleen, liver, and some lymph nodes. In the young children and adults, haematopoiesis occurs in the marrow located in the hollow space of the long bones such as the femur and tibia. Furthermore, the adult`s pelvis, cranium, sternum and vertebral bones play an important role in the production of WBCs. Extramedullary hematopoiesis is 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.

 

Further reading

1.       

Blood and circulatory system

2.

Blood groups

3.

Hemolytic anemias

4.

Blood pressure              

 

Questions

  1. Major proportion of the WBCs are agranulocytes, is it true?

  2. What is the main function of WBCs?

  3. What is haematopoiesis? Where does it take place?

  4. Explain about agranulocytes.

  5. What is the structural difference between granulocytes and agranulocytes?

 

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