NEET'22 Crash Course

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.


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.


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.


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.


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.


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 (Frequently Asked Questions)

1. Why the WBCs are nucleated whereas the RBCs are not?

Ans: WBCs do not have to carry hemoglobin at a high amount for carrying gases. They perform reactions to keep our body protected from infections and cancer cell formation. They do not need extra space to accommodate respiratory pigments as they get changed once they perform their functions.

2. Why do WBCs squeeze out of the blood vessels?

Ans: They reach the tissues affected by any infection or the presence of foreign entities to protect them. Their phagocytic nature enables them to squeeze out of the blood vessels and enter the gaps between the cells to perform their functions.