LEUKOCYTES (WHITE BLOOD CELLS): CHARACTERISTICS, FUNCTIONS, TYPES - BIOLOGY - 2025

The leukocytes are a diverse set of blood cells belonging to the immune system. These lack pigments, which is why they are also known as white blood cells. Cells are involved in the immune response and in the elimination of potential pathogens that enter the body.

White blood cells are divided into two broad categories: granulocytes and mononuclear cells or agranulocytes. Within the granulocytes we have eosinophils, basophils and neutrophils. The contents of the granules are often toxic and when these cells empty them, they can fight infection. The nuclei of these cells are usually segmented or lobed.

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Mononuclear cells are made up of two types of cells: monocytes and lymphocytes. Each type of leukocyte has a particular role in defense.

Most leukocytes originate from a myeloid stem cell, whereas lymphocytes originate from a lymphoid stem cell. When the leukocyte counts are altered, this can be a medical indication to diagnose some pathology or infection.

Historical perspective

The discovery of leukocytes occurred in the mid-18th century by William Hewson, who simply described them as cells devoid of color.

In addition, he mentioned that these cells were produced by the lymphatic system, where they were transported into the bloodstream. Hewson believed that white blood cells could later become red blood cells.

During that time, there were no staining techniques that allowed a more detailed study of leukocytes. For this reason, it was not until the 19th century that researcher Paul Ehrlich used different dyes that allow him to classify white blood cells into different types.

Human immune system: innate and adaptive

To understand the role of leukocytes in the immune system, we must know that this system is clearly divided into two components known as innate and adaptive. Each has its own recognition receptors and has its own speed to respond to pathogens that attack the host.

Innate immunity

The mechanisms that are activated immediately in the presence of a foreign entity correspond to innate immunity. These barriers include the skin and mucosa, soluble molecules such as complement, peptides with antimicrobial properties, cytosines, among others. Evolutionarily, it seems to be a primitive system.

The cells that compose them are macrophages and dendritic cells. These cells use receptors that recognize certain patterns found in genetic material, rapidly responding to general biochemical structures shared between pathogens.

Adaptive immunity

In contrast, the adaptive response is much slower. Within the cells that compose it we have T and B lymphocytes that have receptors for specific antigens. The adaptive response has "memory" and can respond more quickly if the antigen in question has already been present in the body.

These two systems act synergistically with the common goal of defending the body from infection and against the proliferation of cancer cells.

Features and functions

Blood is a liquid tissue that travels its way inside the cardiovascular system. Within this liquid matrix are elements and cell fragments of three types: erythrocytes or red blood cells, leukocytes or white blood cells, and thrombocytes or platelets.

Dimensions and morphology

Leukocytes or white blood cells are a group of spherical cells that lack pigments. Average dimensions vary between 9 and 18 micrometers (µm).

Unlike other blood cells, leukocytes retain the nucleus in its mature cellular state. In fact, the nucleus is the main characteristic used for the classification of these cells.

Features

They are involved in the defense of the organism. Leukocytes have the ability to move through cell spaces through a process called diapédesis, migrating through amoeboid movement.

This mobility is controlled mainly by chemotaxis and by neutrophils. To eliminate pathogens, leukocytes perform phagocytosis.

There are five main types of leukocytes and each one is associated with a particular function within the immune system. As the cells that make up the leukocytes are very heterogeneous among themselves, we will describe their characteristics and functions in detail in the next section.

Types of lymphocytes

There are multiple classifications for leukocytes. The classification can be established by observing the cell under the light of an optical microscope, after being stained with a series of dyes, or they can be classified according to the antigens present on the cell surface using a technique called flow cytometry.

In this article, we will use the classification given by the optical microscope, thanks to its wide use and its simplicity. Below we will describe in detail each of the main categories: granulocytes and mononuclear cells.

Granulocytes

As the name implies, granulocytes are cells whose cytoplasms are rich in granules. In addition to the presence of these compartments, granulocytes are characterized by the presence of lobed or segmented nuclei.

There is a subcategory within granulocytes that classifies cells depending on their response to different colors.

If the granules are stained with acid dyes such as eosin, they are eosinophils. If the dye that stains them is basic in nature, such as methylene blue, the granulocyte is called a basophil. Finally, if it does not respond to the stains, they are called neutrophils.

As the nuclear cleavage of neutrophils is prominent, they are often called polymorphonuclear cells.

Neutrophils

Neutrophils are the most abundant granulocytes and form the first line of defense against infections caused by bacteria and other agents. They are elements of the innate immune system.

Cellular granules have a whole battery of enzymes and bactericides that helps to destroy pathogens and foreign entities.

To fulfill their function, these cells can migrate to different tissues and engulf the element in question. After destroying the pathogen, the neutrophil usually dies and can be excreted together with the bacterial waste in the form of pus.

Neutrophils can secrete a series of substances that alert other cells of the immune system - be it other neutrophils or macrophages - and "call" or recruit them to the site where they are needed.

They are also related to the inflammatory response and the production of extracellular neutrophil traps.

Eosinophils

Within the granulocytes, eosinophils represent only a small percentage of total cells - although their number may increase in patients with infections or fever. They are related to the response to allergy events.

Like neutrophils, eosinophils are white blood cells that can engulf foreign agents that enter the body. They have been specifically related to the presence of parasites and helminths.

The granules presented by eosinophils contain digestive enzymes and other cytotoxic components, allowing them to perform their role as a defense cell.

Although they are cells of very small dimensions to engulf a parasite, eosinophils can settle on the surface of the parasite and empty the toxic content of the granules.

Basophils

In granulocytes, basophils are the least abundant cells. This involves a series of methodological complications to study them, so very little is known about their biology and function.

Historically, basophils were considered cells with a secondary role in allergy processes. This was intuited by the presence of receptors for immunoglobulins E on the membrane surface.

Today it has been possible to confirm the role of basophils as members of the innate and adaptive immune system. These cells are capable of secreting a series of cytokines that help modulate the immune response and also induce B cells to synthesize immunoglobulins E.

Thanks to the release of cytokines, the basophils initiate the allergic reaction. This process is not restricted by specific antigen reactions with immunoglobulins E, they can be triggered by a long list of other molecules such as parasite antigens, lectins, among others.

Unlike eosinophils and neutrophils, the granule content of basophils has been poorly studied.

Along with eosinophils, basophils also play a role in fighting infestations caused by helminths.

Mononuclear cells

The second category of leukocytes are mononuclear cells where we find monocytes and lymphocytes.

Unlike granulocytes, the nucleus of mononuclear cells is not segmented or lobulated, it is rounded and unique. They are also called agranulocytes, because they lack the typical granules of eosinophils, basophils, and neutrophils.

Monocytes

Monocyte characteristics

Monocytes are the largest lymphocytes and in terms of proportions, they account for almost 11% of all circulating leukocytes. They are characterized by exhibiting a kidney-shaped nucleus and a bluish cytoplasm. They exist in both blood and tissues.

Features

The functions of monocytes are quite varied, participating in both innate and adaptive immune system reactions.

As part of the innate immune system, monocytes are capable of recognizing a series of pathogens of a bacterial nature through the recognition of receptors that stimulate cytokine production and phagocytosis.

They have a series of Fc-type receptors, so they can engulf and attack materials that have been coated with antibodies.

Macrophages and dendritic cells can interact with T and B lymphocytes to initiate an adaptive response. Dendritic cells are known for their excellent role as antigen presenting cells.

Lastly, monocytes participate in the removal of cell debris and dead cells in areas where tissue damage or infections have occurred. They also participate in the synthesis of proteins such as coagulation factors, complement components, enzymes, interleukins, among others.

Lymphocytes

Lymphocyte characteristics

Lymphocytes are cells that originate in the bone marrow, where they differentiate and mature. At the end of their development, the cells enter into circulation. The number of leukocytes varies depending on several factors, such as the age, sex, and activity of the person.

Lymphocytes exhibit a couple of peculiarities, when compared to the rest of the leukocytes. They are not terminal cells, since when they are stimulated they begin a process of mitotic cell division, resulting in effector and memory cells.

They have the ability to move from the blood to the tissues, and then back to the blood. Due to the complexity of the process, the migration pattern is not well described in the literature.

Lymphocyte types

These are divided into three large groups: T cells, B cells and natural killer cells or NK (natural killer English). T and B cells play an indispensable role in the adaptive immune response, while NK cells are a small percentage of lymphocytes that participate in the innate response.

T cells are so called because they are produced in the thymus, B cells in the bone marrow (B comes from bone marrow), while NK cells are produced in both sites.

Regarding the adaptive response, there are three characteristics that we must highlight. First, it has a significantly high number of lymphocytes, each with specific receptors located on its membranes that recognize specific sites for foreign antigens.

After having contact with an antigen, the cell can remember it and this cellular memory can cause a faster and more vigorous reaction if there is a re-exposure to the same antigen. Note that antigens from the body are tolerated and ignored by the immune system.

Lymphocyte functions

Each type of lymphocyte has a specific function. B lymphocytes participate in the production of antibodies and in the presentation of antigens to T cells.

B cells are also involved in the production of cytokines that regulate a variety of T cells and antigen presentation.

T cells divide into CD4 + and CD8 +. The former fall into multiple categories and participate specifically in functions such as mediating the immune response against intracellular pathogens, bacterial infections, fungal induction of asthma, and other allergic responses.

Those of the CD8 + type are capable of destroying target cells through the secretions of granules that contain a series of toxic enzymes. In the literature, CD8 + cells are also known as cytotoxic T lymphocytes, for all the molecules that they release.

The function of NK lymphocytes is directly linked to the innate immune response. In addition, they are capable of killing tumor cells and cells that are infected by viruses. Additionally, NK cells can modulate the functions of other cells, including macrophages and T cells.

Half-life of leukocytes

Granulocytes and monocytes

The life of the leukocyte in the bloodstream or in the tissues depends on the type studied. Some granulocytes, such as basophils, live for only a few hours and eosinophils live for a few days, about a little over a week. Monocytes also last from hours to days.

Lymphocytes

The life span of lymphocytes is notably longer. Those that are involved in memory processes can last for years and those that do not last a few weeks.

Diseases

Normal leukocyte values ​​are in the order of 5 to 12.10 ³ per mL. Alterations in the total leukocyte count are known as leukopenia and leukocytosis. The first term refers to a low number of cells, while leukocytosis refers to a high number.

Leukocytosis

A high number of leukocytes can occur due to a response in the body to a wide range of physiological or inflammatory processes, the latter being the most frequent cause. Inflammatory or infectase leukocytosis occurs due to the presence of bacteria, viruses, and parasites.

Depending on the infectious agent, the levels of specific leukocytes vary in a particular way. That is, each pathogen raises a certain type of leukocyte.

For example, if the agent is a virus, there may be leukopenia or leukocytosis. In the case of bacteria, the initial infection is characterized by neutrophilia, then by monocytosis, and ends with lymphocytosis and reappearance of eosinophils.

An increase in neutrophils may indicate an inflammatory response. An increase in the eosinophil count is related to the presence of parasites or a hypersensitivity event.

The last type of leukocytosis is of the non-infectious type, and can occur from neoplastic or non-neoplastic and non-hematologic hematologic causes.

Knowing that the leukocyte values ​​are abnormal is not really very informative. The type of cell that is affected must be characterized in order to establish a more precise diagnosis.

Leukopenia

A low number of leukocytes in the patient can occur due to a decrease in their production in the bone marrow, hypersplenism, among other conditions. Leukocytes are considered to be in low abnormal numbers if the number is less than 4,000 leukocytes per mm ³.

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