Leukocytes

Edited By Irshad Anwar | Updated on Jul 02, 2025 07:26 PM IST

What Are Leukocytes (White Blood Cells)?

Leukocytes are more popularly known as white blood cells (WBCs). They form an integral part of the immune system and play a very important role in body defence against infections, foreign invasions, and diseases. In contrast to erythrocytes, leukocytes are not restricted to the blood. They move in and out of blood vessels into the tissues to perform their functions wherever necessary.

This Story also Contains

What Are Leukocytes (White Blood Cells)?
Structure Of Leukocytes
Types Of Leukocytes
Functions Of Leukocytes
Role Of Neutrophils :
Role Of Lymphocytes
Role Of Monocytes
Leukocyte Production
Disorders Of Leukocytes
Role Of Leukocytes In Health
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Structure Of Leukocytes

Nucleus: Unlike erythrocytes, the leukocyte does have a nucleus. It is considered an essential component because, without a nucleus, the leukocytes would not have the ability to function and reproduce.
Cytoplasm: The cytoplasm has many granules that assist in performing their immune functions.
Cell Membrane: The cell membrane is semipermeable and pliable. This allows the leukocyte to change shape and move through tissue.

Types Of Leukocytes

There are two general types of leukocytes:

Granulocytes: These include neutrophils, eosinophils, and basophils. They have granules in the cytoplasm and are involved in inflammatory reactions and protection from infection.
Agranulocytes: They are made of lymphocytes and monocytes. They play a part in specific immune responses and antibody production.

Functions Of Leukocytes

Defence Against Infections: Leukocytes recognize the disease-causing organisms like bacteria, viruses, and fungi, and destroy them.
Immune Response Regulators: They act as immune response regulators by displaying signals that communicate with other immune cells and instruct them in their actions.
Inflammation: The inflammatory response in which leukocytes take part helps to isolate and clear the pathogenic agents.

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Role Of Neutrophils :

Neutrophils are the most common granulocytes and can often be the first to arrive at the site of infection.
They internalize and digest pathogens through the process of phagocytosis.
Their rapid response and capacity for creating reactive oxygen species make them critical during the early natural phase of immune defence.

Role Of Lymphocytes

Lymphocytes play a central role in adaptive immunity. There are two major types:

B Lymphocytes: These cells produce antibody proteins that recognize specific pathogens.
T Lymphocytes: These cells can, in a process called apoptosis, directly kill infected cells, or they help to control the immune response.

Role Of Monocytes

Monocytes are large WBCs that mature into macrophages and dendritic cells when they enter tissues.
As mentioned earlier, macrophages phagocytize and present antigens to T lymphocytes.
Dendritic cells also present antigens to T lymphocytes.
Furthermore, dendritic cells sitios activate the adaptive immune response.

Leukocyte Production

Hematopoiesis: Leukocytes develop in the bone marrow via a process known as hematopoiesis that entails the differentiation of hematopoietic stem cells.
Lifespan: The life span of leukocytes is pretty variable. For example, neutrophils may live from a few hours to a few days and memory lymphocytes might persist even for years.

Disorders Of Leukocytes

Leukaemia: It is a kind of cancer that is marked by the uncontrolled multiplication of abnormal leukocytes.
Leukopenia: A low count of white blood cells; increases the susceptibility to infections.
Lymphoma: A cancer of the lymphatic system that affects lymphocytes.

Role Of Leukocytes In Health

Leukocytes are essential in maintaining good health.
The cells help in fighting infections and diseases.
Because of their ability to amend their shape and phagocytose most pathogens, they become part of the immune system.
WBC count may, in turn, be observed to give a related fact about one's health status.

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Frequently Asked Questions (FAQs)

1. What are leukocytes?

The cells of the immune system that constitute the white blood cells fighting against infection and disease in the body are called leukocytes.

2. What are the major types of leukocytes?

The major types of leukocytes include granulocytes.

3. How are the leukocytes produced?

Leukocytes are produced in the bone marrow via a process called hematopoiesis.

4. What is the function of neutrophils?

Neutrophils constitute the first line of defence against pathogens. They phagocytose and digest them.

5. What is leukaemia?

Leukemia is a type of cancer that is characterized by the excessive production of abnormal leukocytes.

6. What is leukocytosis and what can cause it?

Leukocytosis is an increased number of leukocytes in the blood, typically above 11,000 cells per microliter. It's often a sign that the body is fighting an infection or inflammation. Common causes include bacterial or viral infections, inflammation (like in rheumatoid arthritis), certain medications (like corticosteroids), stress, and some types of cancer (particularly leukemia). The specific type of leukocyte that's elevated can provide clues about the underlying cause.

7. What is leukopenia and what are its potential causes?

Leukopenia is a decrease in the number of leukocytes in the blood, typically below 4,500 cells per microliter. It can be caused by various factors, including certain viral infections (like HIV), autoimmune disorders, bone marrow diseases, radiation exposure, chemotherapy, and some medications. Leukopenia can make a person more susceptible to infections, as there are fewer white blood cells available to fight off pathogens.

8. What is the significance of the differential white blood cell count?

The differential white blood cell count measures the relative percentages of different types of leukocytes in the blood. This test is important because changes in the proportions of different leukocytes can indicate specific health conditions. For example, a high percentage of neutrophils might suggest a bacterial infection, while elevated lymphocytes could indicate a viral infection or certain types of leukemia. The differential count helps doctors diagnose diseases and monitor the effectiveness of treatments.

9. What is the normal range of leukocyte count in blood?

The normal range for leukocyte count in adults is typically between 4,500 and 11,000 cells per microliter of blood. However, this range can vary slightly depending on factors like age, sex, and the specific laboratory standards. A count higher than this range (leukocytosis) may indicate infection or inflammation, while a lower count (leukopenia) might suggest bone marrow problems or certain diseases.

10. How do eosinophils and basophils contribute to allergy responses?

Eosinophils and basophils are involved in allergic responses and fighting parasitic infections. During an allergic reaction, these cells release inflammatory mediators like histamine, which cause typical allergy symptoms such as itching, swelling, and mucus production. Eosinophils also release toxic proteins that can kill parasites but may damage surrounding tissues in allergic conditions.

11. How do leukocytes differ from erythrocytes?

Leukocytes (white blood cells) and erythrocytes (red blood cells) differ in several ways. Leukocytes are larger, have a nucleus, and are fewer in number compared to erythrocytes. While erythrocytes primarily transport oxygen, leukocytes are responsible for immune functions. Leukocytes can move independently and can leave blood vessels to enter tissues, whereas erythrocytes typically remain within blood vessels.

12. How do leukocytes respond to viral infections?

Leukocytes respond to viral infections through various mechanisms. Innate immune cells like neutrophils and macrophages can engulf virus-infected cells and produce antiviral cytokines. Natural killer cells can directly kill virus-infected cells. In the adaptive response, cytotoxic T cells recognize and destroy infected cells, while helper T cells coordinate the overall immune response. B cells produce antibodies that can neutralize viruses and prevent them from infecting cells. The coordinated action of these leukocytes is essential for effectively combating viral infections.

13. How do leukocytes interact with the complement system?

Leukocytes interact closely with the complement system, a part of the innate immune response. Complement proteins can opsonize (coat) pathogens, making them easier for phagocytic leukocytes like neutrophils and macrophages to engulf. Complement also produces chemical signals that attract leukocytes to sites of infection (chemotaxis). Some leukocytes, particularly B cells and certain T cells, can activate the complement system. In turn, complement proteins can enhance various leukocyte functions, creating a synergistic relationship that amplifies the overall immune response.

14. How do leukocytes contribute to allergic reactions?

Leukocytes play central roles in allergic reactions. When an allergen is encountered, it triggers a complex immune response involving several types of leukocytes. B cells produce IgE antibodies specific to the allergen. These antibodies bind to receptors on mast cells and basophils, which then release histamine and other inflammatory mediators when re-exposed to the allergen. Eosinophils are also recruited and release substances that can damage tissues. T helper cells, particularly Th2 cells, coordinate this allergic response by producing cytokines that promote IgE production and eosinophil activation.

15. What is the role of leukocytes in transplant rejection?

Leukocytes are key players in transplant rejection. When an organ is transplanted, the recipient's immune system, particularly T lymphocytes, can recognize the donor tissue as foreign due to differences in major histocompatibility complex (MHC) molecules. This triggers an immune response where cytotoxic T cells directly attack the transplanted tissue, while helper T cells coordinate a broader immune response involving other leukocytes and antibodies. Macrophages and neutrophils can also contribute to tissue damage. Understanding and modulating these leukocyte responses is crucial for preventing transplant rejection.

16. What is the significance of leukocyte telomere length?

Telomeres are protective structures at the ends of chromosomes that shorten with each cell division. Leukocyte telomere length is often used as a biomarker of cellular aging and overall health. Shorter leukocyte telomeres have been associated with various age-related diseases and conditions, including cardiovascular disease and certain cancers. The length of leukocyte telomeres can be influenced by factors like stress, lifestyle, and environmental exposures. Studying leukocyte telomere length provides insights into the aging process and how it affects immune function.

17. How do leukocytes move to sites of infection?

Leukocytes move to infection sites through a process called chemotaxis. When tissues are damaged or infected, they release chemical signals called chemokines. Leukocytes detect these signals and move towards higher concentrations of chemokines. They can then leave blood vessels through a process called diapedesis, squeezing between endothelial cells to enter infected tissues.

18. How do leukocytes recognize foreign invaders?

Leukocytes recognize foreign invaders through various mechanisms. Some leukocytes, like neutrophils and macrophages, have receptors that can identify common molecular patterns found on many pathogens. Other leukocytes, particularly lymphocytes, use highly specific receptors to recognize unique molecules (antigens) on pathogens. This recognition triggers an immune response to neutralize or destroy the invader.

19. How do leukocytes communicate with each other?

Leukocytes communicate with each other and other cells through chemical messengers called cytokines. These small proteins can trigger various responses in target cells, such as cell activation, proliferation, or migration. Different types of leukocytes produce different cytokines, creating a complex network of communication that coordinates immune responses. This intercellular communication is crucial for mounting an effective and regulated immune response.

20. What is the process of leukocyte extravasation?

Leukocyte extravasation, also known as diapedesis, is the process by which leukocytes leave the bloodstream and enter tissues. It involves several steps: rolling (leukocytes slow down and roll along the blood vessel wall), adhesion (they stick to the vessel wall), crawling (they move along the vessel wall to find an exit point), and transmigration (they squeeze between endothelial cells to exit the blood vessel). This process is crucial for leukocytes to reach sites of infection or inflammation in tissues.

21. How do leukocytes contribute to inflammation?

Leukocytes are key players in the inflammatory response. When tissue is damaged or infected, leukocytes (particularly neutrophils and macrophages) are recruited to the site. They release inflammatory mediators that increase blood flow and vascular permeability, allowing more immune cells to enter the area. These leukocytes also engulf pathogens and cellular debris, release antimicrobial substances, and produce cytokines that further coordinate the immune response. While inflammation is a protective mechanism, excessive or chronic inflammation can lead to tissue damage.

22. How do lymphocytes contribute to adaptive immunity?

Lymphocytes, including T cells and B cells, are key players in adaptive immunity. B cells produce antibodies that can recognize and neutralize specific pathogens. T cells have various roles: helper T cells coordinate immune responses, cytotoxic T cells directly kill infected cells, and regulatory T cells help control immune responses. Memory cells, a subset of both B and T cells, provide long-lasting immunity against previously encountered pathogens.

23. What is the role of natural killer cells among leukocytes?

Natural killer (NK) cells are a type of lymphocyte that plays a crucial role in the innate immune system. Unlike other lymphocytes, NK cells can recognize and destroy virus-infected cells or tumor cells without prior sensitization. They do this by detecting cells that have reduced expression of MHC class I molecules, a common feature of infected or cancerous cells. NK cells also produce cytokines that help coordinate broader immune responses.

24. How do memory B and T cells contribute to long-term immunity?

Memory B and T cells are specialized lymphocytes that persist in the body long after an initial immune response. These cells "remember" specific pathogens and can quickly recognize and respond to them if they're encountered again. Memory B cells can rapidly produce antibodies, while memory T cells can quickly proliferate and carry out their specific functions. This allows for a faster and more effective immune response upon re-exposure to a pathogen, forming the basis of long-term immunity and the effectiveness of vaccines.

25. What is the role of regulatory T cells in the immune system?

Regulatory T cells (Tregs) are a specialized subpopulation of T cells that act to suppress immune responses and maintain immune system homeostasis. They play a crucial role in preventing autoimmune diseases by suppressing self-reactive T cells that have escaped other mechanisms of tolerance. Tregs also help to resolve inflammation and prevent excessive immune responses that could damage tissues. The balance between regulatory and effector T cells is critical for maintaining a healthy immune system.

26. How do leukocytes contribute to autoimmune diseases?

In autoimmune diseases, the immune system mistakenly attacks the body's own tissues. Leukocytes, particularly lymphocytes, play a central role in this process. In these conditions, T cells may recognize self-antigens as foreign, leading to their activation and the stimulation of B cells to produce autoantibodies. Other leukocytes, like neutrophils and macrophages, can contribute to tissue damage through the release of inflammatory mediators. Understanding how these leukocyte responses become dysregulated is key to developing treatments for autoimmune diseases.

27. What is the function of monocytes?

Monocytes are large leukocytes that circulate in the blood for a few days before entering tissues, where they differentiate into macrophages or dendritic cells. As macrophages, they engulf and destroy pathogens and cellular debris through phagocytosis. They also present antigens to T cells, initiating adaptive immune responses. Monocytes and macrophages play crucial roles in both innate and adaptive immunity.

28. What is the role of dendritic cells in the immune system?

Dendritic cells are specialized antigen-presenting cells that bridge innate and adaptive immunity. They patrol tissues, capturing antigens from pathogens or damaged cells. Once activated, dendritic cells migrate to lymph nodes where they present these antigens to T cells, initiating adaptive immune responses. Dendritic cells also produce cytokines that help shape the type of immune response generated. Their ability to activate and direct T cell responses makes dendritic cells crucial for initiating effective adaptive immunity.

29. How do leukocytes contribute to wound healing?

Leukocytes play several important roles in wound healing. Neutrophils are among the first cells to arrive, cleaning the wound by phagocytosing debris and pathogens. Macrophages follow, continuing the cleaning process and releasing growth factors that stimulate tissue repair. They also help coordinate the later stages of healing. T lymphocytes regulate the healing process and help prevent excessive scarring. The coordinated actions of these leukocytes are essential for proper wound healing and tissue regeneration.

30. How do leukocytes contribute to the development of atherosclerosis?

Leukocytes play a significant role in the development of atherosclerosis, a condition where arteries become hardened and narrowed due to plaque buildup. Monocytes enter the arterial wall and differentiate into macrophages, which then take up oxidized LDL cholesterol and become foam cells. T lymphocytes also enter the arterial wall and produce inflammatory cytokines. This chronic inflammatory process, driven by these leukocytes, contributes to plaque formation and progression. Understanding this process is crucial for developing strategies to prevent and treat cardiovascular diseases.

31. What is the significance of the blood-brain barrier for leukocytes?

The blood-brain barrier (BBB) is a highly selective semipermeable border that separates the circulating blood from the brain extracellular fluid. Under normal conditions, the BBB restricts the passage of most leukocytes into the central nervous system, protecting the brain from potentially harmful immune responses. However, in certain conditions like infections or autoimmune diseases, leukocytes can cross the BBB. Understanding how leukocytes interact with and cross the BBB is important for treating neurological disorders and brain infections.

32. What is the role of neutrophils in the immune response?

Neutrophils are the most abundant type of leukocyte and are crucial in the innate immune response. They are often the first cells to arrive at the site of infection. Neutrophils engulf and destroy pathogens through a process called phagocytosis. They also release antimicrobial substances and can form neutrophil extracellular traps (NETs) to catch and kill pathogens outside the cell.

33. What is the significance of leukocyte adhesion deficiency?

Leukocyte adhesion deficiency (LAD) is a rare genetic disorder where leukocytes, particularly neutrophils, cannot adhere to blood vessel walls and migrate into tissues. This is due to defects in adhesion molecules on the leukocyte surface. As a result, patients with LAD suffer from recurrent bacterial infections because their neutrophils cannot reach infection sites. LAD illustrates the critical importance of leukocyte adhesion and migration in immune function and highlights how genetic defects in these processes can lead to severe immunodeficiency.

34. How do leukocytes contribute to sepsis?

Sepsis is a life-threatening condition

35. What are the main types of leukocytes?

There are five main types of leukocytes: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Each type has specific functions in the immune response. Neutrophils are the most abundant and are first responders to infections. Lymphocytes include T cells and B cells, which are crucial for adaptive immunity. Monocytes become macrophages in tissues and help clean up cellular debris. Eosinophils and basophils are involved in allergic responses and fighting parasites.

36. How do granulocytes differ from agranulocytes?

Leukocytes are divided into granulocytes and agranulocytes based on the presence or absence of visible granules in their cytoplasm. Granulocytes (neutrophils, eosinophils, and basophils) have visible granules containing enzymes and other proteins used in immune responses. Agranulocytes (lymphocytes and monocytes) lack these visible granules. Granulocytes are typically involved in innate immunity, while agranulocytes play key roles in both innate and adaptive immunity.

37. What are leukocytes and why are they important?

Leukocytes, also known as white blood cells, are a crucial component of the immune system. They protect the body against infections and diseases by identifying and destroying harmful pathogens like bacteria, viruses, and parasites. Leukocytes are produced in the bone marrow and circulate throughout the body in blood and lymph, acting as the body's defense mechanism.

38. What is the role of leukocytes in cancer immunosurveillance?

Leukocytes are crucial in cancer immunosurveillance, the process by which the immune system detects and eliminates cancerous or precancerous cells. Natural killer cells and cytotoxic T lymphocytes can directly kill tumor cells. Macrophages and dendritic cells can present tumor antigens to T cells, initiating adaptive immune responses against cancer. However, tumors can develop mechanisms to evade or suppress these immune responses. Understanding these interactions between leukocytes and cancer cells is fundamental to developing effective immunotherapies for cancer treatment.