What Are Leukocytes?
Our immune system is made of white blood cells (WBCs) circulating throughout the body which keep an eye on the foreign entities entering our system. The cellular components of our blood circulatory system suspended in the plasma that has prominent nuclei and motility are called leukocytes. They are also called white blood cells (WBCs) as they do not have pigments like red blood cells (RBCs). The prime function of these cells is to maintain the immune system of our body. Here, we will find out the different types of leukocytes, their origin and functions.
WBCs are differentiated according to their morphological features.
Granulocytes (basophils, neutrophils, and eosinophils)
Agranulocytes (monocytes and lymphocytes)
The granulocytes are also called polymorphonuclear leukocytes as they contain multilobed nuclei. This feature helps in differentiating the WBCs considering the sacs and granules present in their cytoplasm. All these WBCs originate in the bone marrow of long bones. The levels of these cells are regulated by the major organs such as kidneys, spleen, and liver.
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Types of Leukocytes
Now that we know what is leukocytes, let us find out their different types according to their morphological features.
Neutrophils
This is a type of granulocyte with a multi-lobed single nucleus. This type of WBC exists in the highest amount when compared to the other types. When stained for identification, this cell shows a neutral pink color. This is why it is called neutrophils.
These cells have a strong affinity towards pathogenic bacteria. They can easily migrate from the circulatory system to the adjacent tissues fighting an infection. Their motility enables them to squeeze through the cells and kill the bacteria present in the tissues by engulfing and digesting them.
The granules present inside these cells are lysosomal sacs that release digestive enzymes to kill the engulfed bacteria. In fact, neutrophils are also called suicidal cells as they also get destroyed in the process of phagocytosis and internal digestion.
Basophils
As the name suggests, these granulocytic types of leukocytes show a distinguishable dark blue stain due to the presence of basic granular sacs. A basophil contains a multi-lobed nucleus. Its prime function is to release biochemical compounds to combat allergic reactions. This type of WBC is present the highest in number.
The granules inside these cells contain immune-boosting biochemical compounds heparin and histamine. Heparin is a natural anticoagulant whereas histamine is a blood vessel dilator. Histamine functions as a dilator of vessels to increase blood flow and to escalate the level of permeability for the passage of other WBCs to the infected tissues.
Eosinophils.
This type of granulocytic WBC shows an acidic bright red eosinophilic stain and is called an eosinophil. The leukocytes function of these cells is to destroy parasites and develop cancer cells in the entire body. These WBCs also help in fighting allergic reactions.
The nucleus of these cells is U-shaped. These cells are mostly found in the connective tissues of the abdomen, intestines, and stomach. They are phagocytic in nature and generally target the complexes formed from binding antibodies with antigens. These cells are reactive to the targets that an antibody marks by combining with the antigen present on the parasitic cells.
Monocytes
These agranular cells are the biggest in size among all the WBCs and are mononuclear in nature. They are generally round in shape but can appear in different shapes. These cells are also phagocytic in nature and can migrate from the blood vessels to the nearest tissues for fighting infections.
The prime leukocytes function of these cells is to protect the tissues of the vital organs with its phagocytic action. Once they enter into tissues between the cells, they either become dendritic cells or macrophages.
Dendritic cells reside in the tissues that mostly come in contact with the antigens such as lungs, skin, nose, and gastrointestinal tract. These cells send information to the nearest lymph glands for the development of immunity.
Macrophages, on the other hand, can be found in all the tissues in our body. These leukocytes assist in the development of hormones and vital blood vessels around the ovaries during pregnancies.
Lymphocytes
This is also another type of agranulocyte WBC assisting the immune system of a human being. It has two types, lymphocyte B and lymphocyte T. B lymphocytes generate antibodies whereas T lymphocytes identify tumor cells and eradicate them. It also assists in controlling the immune response of an individual.
Difference Between Erythrocytes and Leukocytes
Erythrocytes are called red blood cells (RBCs). These cells are non-nucleated blood cells suspended in the plasma and accommodate hemoglobin as a respiratory pigment. Unlike the WBCs, they are red in color and remain inside the blood vessels. They transport oxygen and carbon dioxide to and from the lungs and tissues throughout the body.
RBCs are smaller in size than WBCs. They live for 120 days whereas the average lifespan of WBCs is 12-20 days. They do not take part in immune responses, rather aid in the coagulation of blood in the ruptured vessels to stop bleeding. There is only one type of RBC in the blood.
This is the detailed explanation of leukocytes present in our circulatory system and their functions. The characteristic features and functions differentiate these cells creating 5 types of leukocytes, responsible for various immunological responses.
FAQs on Leukocytes
1. What are leukocytes?
Leukocytes, more commonly known as White Blood Cells (WBCs), are a crucial component of the blood and the body's immune system. Unlike red blood cells, they are nucleated and function primarily to identify and destroy pathogens like bacteria and viruses, remove dead cells, and protect against diseases.
2. What are the main types of leukocytes and their functions?
Leukocytes are broadly classified into two main categories based on the presence of granules in their cytoplasm:
- Granulocytes: These have visible granules and include Neutrophils (phagocytose bacteria), Eosinophils (combat parasitic infections and allergic reactions), and Basophils (release histamine during inflammation).
- Agranulocytes: These lack visible granules and include Lymphocytes (B-cells and T-cells, crucial for specific immunity) and Monocytes (which mature into macrophages to engulf pathogens and cellular debris).
3. What is the normal range for leukocytes in a healthy adult?
In a healthy adult, the normal total leukocyte count (TLC) typically ranges from 4,000 to 11,000 cells per microliter (µL) of blood. This count can fluctuate based on factors like age, stress, and the presence of infection. A significant deviation from this range often indicates an underlying medical condition.
4. What is the clinical importance of having a high leukocyte count?
A high leukocyte count, a condition known as leukocytosis, is most commonly a sign that the body is fighting an infection. The immune system increases the production of WBCs to combat invading pathogens. However, persistently high levels can also be associated with chronic inflammation, stress, certain medications, or more serious conditions like leukaemia.
5. Why are mature red blood cells anucleated while leukocytes retain their nucleus?
Red blood cells (RBCs) lose their nucleus during maturation to maximise intracellular space for haemoglobin, the protein that carries oxygen. This specialisation makes them highly efficient oxygen transporters. Leukocytes, on the other hand, retain their nucleus because it contains the genetic material essential for directing their complex immune functions, such as producing antibodies, signalling other cells, and engaging in phagocytosis. The nucleus acts as their functional command centre.
6. How do leukocytes leave the bloodstream to reach sites of infection in the body?
Leukocytes reach infection sites through a remarkable process called diapedesis. In response to chemical signals from an infected or inflamed area, these cells change shape and become amoeboid. This allows them to actively squeeze through the tiny gaps between the endothelial cells that form the walls of blood capillaries, enabling them to enter the affected tissues and fight pathogens directly.
7. What is the key difference in function between granulocytes and agranulocytes?
The primary functional difference lies in their roles within the immune system. Granulocytes (neutrophils, eosinophils, basophils) are part of the innate immune system, providing a rapid, non-specific first line of defence. Agranulocytes (lymphocytes and monocytes) are key players in the adaptive immune system, responsible for mounting a highly specific, targeted attack against particular pathogens and forming long-term immune memory.
8. What does the presence of leukocytes in urine typically indicate?
Finding leukocytes in urine, a condition called pyuria, generally indicates inflammation or infection somewhere in the urinary tract, such as a urinary tract infection (UTI). The presence of these immune cells shows that the body has mounted a defence against bacteria or other pathogens in the bladder, urethra, or kidneys.
