What are the 5 types of white blood cells and their functions. The following article discusses in detail the 5 types of white blood cells and what are their functions in the body in terms of the immune system.
When it comes to the immune system, white blood cells (WBCs), or leukocytes, act as the second line of defense. What does this mean? When pathogens breach the first line of defense, also known as physical barriers, and enter the bloodstream, white blood cells, along with other internal defense mechanisms, attack the pathogens and prevent them from spreading further into the body.
Types of White Blood Cells (WBCs)
White Blood Cells (WBCs) are colorless because they lack hemoglobin. They are nucleated and are comparatively fewer in number than red blood cells (RBCs). The average WBC count ranges between 6,000 and 8,000 per cubic millimeter of blood, and they are short-lived. However, their number increases in response to infections. An increase in the WBC count is called leukocytosis.
When WBCs squeeze out of the capillaries to fight infection, it is called diapedesis. Among all WBCs, diapedesis is mainly exhibited by neutrophils, as they are smaller in size and most numerous. If you are asked why RBCs, though smaller in size, cannot show diapedesis, the answer is simple: RBCs are not capable of amoeboid movement, as they cannot change their shape like amoebas.
WBCs are mainly of two types:
. Granulocytes
. Agranulocytes
Granulocytes: WBCs where cytoplasmic granules are present are called granulocytes. These cells have enzyme-rich specific granules. These granules are formed by lysosomes. Granulocytes comprise of 68% of total WBCs. Example: Basophils, Acidophils (Eosinophils), Neutrophils.
These WBCs have been classified based on their staining properties. For instance, neutrophils can be stained with acid and basic dyes. Acidophils are stained with acidic dyes and basophils are stained with basic dyes.
Neutrophils
- Neutrophils are the most abundant cells of the total WBCs.
- Neutrophils comprise of 60-65% of the total WBCs.
- Neutrophils can be stained with both acidic and basic dyes.
- Neutrophils have multilobed nucleus and they can be present in different forms. For instance, if an individual’s blood sample is taken and viewed under the microscope, many different types of neutrophils can be seen. For instance, some cells will have 5 nuclear lobes, others will have 4, 7, and 6 respectively. Due to this nature of neutrophils, they are also called as Poly Morpho Nuclear Leucocytes (PMNLs).
- They are called primary phagocytic cells, also known as Micro Policeman of Blood. They are called primary phagocytic cells because neutrophils and monocytes both are phagocytic in nature. Due to large number of neutrophils, they are the first to attack pathogens and thus called primary phagocytes.
Eosinophils / Acidophils
- The nucleus of eosinophils is bilobed
- They are stained with acidic dyes
- These resist infections and are also associated with allergic reactions
- Eosinophiles are most effective against large-sized parasites such as blood fluke
- These secrete histamine (vasodilator)
Basophils
- Basophils are the least in number among all WBCs (least abundant)
- They are stained with basic dyes
- The nucleus of basophils is S-shaped
- Basophils secrete Histamine (vasodilator), Serotonin (vasoconstrictor), and Heparin (anticoagulant)
Agranulocytes: WBCs where no cytoplasmic granules present. In other words, they lack enzyme-rich specific granules. Agranulocytes comprise of 32% of the total WBCs. Example: Monocytes and Lymphocytes.
Monocytes
- Monocytes are the largest cells among all WBCs
- Monocytes are most efficient phagocytes. Though neutrophils are also phagocytic in nature, but monocytes are very efficient.
- Monocytes are recognized by their horse-shoe shaped or kidney shaped nucleus
- Monocytes are also called as macrophages because they are larger in size and phagocytic in nature.
- Macrophages function as antigen presenting cells (APCs)
- Macrophages are of two types: fixed macrophages and wandering macrophages. Fixed macrophages are attached to tissues while wandering macrophages can move from one place to another. Examples of fixed macrophages are: dust cells (lungs), Lacis cells (kidney), Microglial cells (brain), Histiocytes (connective tissue)
Lymphocytes
- Lymphocytes are the main cells of the immune system.
- Lymphocytes are of two types: B-lymphocytes and T-lymphocytes
- In B-lymphocytes, B stands for Bursa of Fabricius – a lymphatic gland in the cloaca of young birds, where it was first discovered by scientists. Bursa of Fabricius is not found in humans. In case of humans, B stands for Bone Marrow.
- In T-lymphocytes, T stands for Thymus.
Development of Lymphocytes
The long bones of the body, the epiphysis region consists of bone marrow (also known as the primary lymphatic organ). Within the bone marrow, a special kind of cell is present, called hematopoietic stem cells (HSCs). These hematopoietic stem cells differentiate into lymphoid stem cells and myeloid stem cells.
Lymphoid stem cells give rise to two different lymphatic cells, called Pre-B cells and Pre-T cells. Pre-B cells from the primary lymphatic organ (bone marrow) enter into the secondary lymphatic tissues (spleen, lymph nodes, tonsils, Peyer’s patches), where they further differentiate into Memory B cells and Plasma Cells.
Pre-T cells, on the other hand, enter the thymus (a primary lymphoid organ), where they mature and differentiate into three types of cells: TC (Cytotoxic T cells), TH (Helper T cells), and TK (Killer T cells).
Myeloid stem cells are responsible for the formation of different kinds of cells, including RBCs (red blood cells), platelets, neutrophils, eosinophils, basophils, and monocytes.
Note: Lymphoid stem cells are only responsible for the formation of lymphocytes and not all kinds of white blood cells.