BLOOD CELLS: TYPES, CHARACTERISTICS AND FUNCTIONS - BIOLOGY - 2025

The blood cells are a diverse set of cells that are found circulating in the specialized connective tissue known as blood. These include red cells, white cells, lymphocytes, megakaryocytes, platelets, and mast cells.

These cells are produced during the life of an organism from another group of “rare” pluripotent cells found in the bone marrow and known as hematopoietic stem cells.

Diagram of three types of blood cells: red cells, white cells and platelets (Source: Cancer Research UK via Wikimedia Commons)

Hematopoietic stem cells are characterized by two fundamental aspects: they give rise to new hematopoietic stem cells (self-renewal) and they differentiate into progenitor cells that subsequently become involved in the different hematopoietic lineages.

The hematopoietic system is formed from the embryonic mesoderm and, in vertebrates, the formation of blood cells or hematopoiesis occurs in the embryo sac during the first stages and in the bone marrow throughout adult life.

The formation of blood cells occurs as follows: hematopoietic stem cells give rise to two groups of precursors that can progress to the development of lymphoid or myeloid lineages.

The lymphoid lineage forms the precursors of lymphocytes. T-lymphocyte precursor cells, which arise from precursor cells of the lymphoid lineage, give rise to T cells, and the same is true for B-lymphocyte precursors and cells of the same name.

In the same way, the myeloid lineage gives rise to two groups of progenitor or precursor cells: the Granulocyte / Macrophage precursors and the Megakaryocyte / Erythrocyte precursors. From the former arise monocytes and neutrophils, and from the latter arise erythrocytes and megakaryocytes.

Types

Blood cells are very diverse both in size and shape and in function. There are usually 4 types of cells in the blood: (1) red cells or erythrocytes, (2) white cells or leukocytes (divided into granulocytes and agranulocytes), (3) megakaryocytes and platelets, and (4) mast cells.

Red cells or erythrocytes

Erythrocytes are a type of blood cell with a very important function, since they are responsible for the transport of oxygen throughout the body.

They are cells without internal organelles, with the shape of biconcave discs of around 8μm in diameter and 2μm wide. The shape and characteristics of their membrane make these cells powerful vehicles for gas exchange, since they are rich in various transmembrane transporters.

Inside, the cytosol is full of soluble enzymes such as carbonic anhydrase (which catalyzes the formation of carbonic acid from carbon dioxide and water), all the enzymes of the glycolytic pathway and of the pentose phosphate. These substances are used for the production of energy in the form of ATP and reducing power in the form of NADP +.

One of the most important enzymes in these cells is hemoglobin. This is capable of binding to molecular oxygen and releasing carbon dioxide or vice versa, depending on the surrounding oxygen concentration, which gives the erythrocyte the ability to transport gases through the body.

White cells

White cells, white blood cells, or leukocytes are less abundant than erythrocytes in blood tissue. They use the torrent as a vehicle for transport through the body, but do not reside in it. In general, they are responsible for protecting the body from foreign substances.

White blood cells are classified into two groups: granulocytes and agranulocytes. The former are classified according to the color they acquire in a type of stain known as Ramanovsky stain (neutrophils, eosinophils and basophils) and agranulocytes are lymphocytes and monocytes.

Granulocytes

Neutrophils

Neutrophils or polymorphonuclear leukocytes are the most abundant cells among white blood cells and the first to appear during acute bacterial infections. They are specialized in phagocytosis and bacterial lysis, and participate in the initiation of inflammatory processes. That is, they participate in the nonspecific immune system.

They measure about 12μm in diameter and have a single nucleus with a multilobular appearance. Inside there are three classes of granules: small and specific, azurophiles (lysosomes) and tertiary. Each of these is armed with a set of enzymes that allow the neutrophil to perform its function.

These cells travel through the bloodstream to the endothelial tissue near their destination, which they pass through thanks to the interaction between ligands and specific receptors on the surface of neutrophils and endothelial cells.

Once in the connective tissue in question, neutrophils engulf and hydrolyze invading microorganisms through a series of complex enzymatic processes.

Eosinophils

These cells represent less than 4% of the white blood cells. They are responsible for the phagocytosis of antigen-antibody complexes and various invading parasitic microorganisms.

They are round cells (in suspension) or pleomorphic (with different shapes, during their migration through connective tissue). They have a diameter between 10 and 14μm and some authors describe them in the shape of a sausage.

They have a bilobed nucleus, a small Golgi complex, few mitochondria, and a reduced rough endoplasmic reticulum. They are produced in the bone marrow and are capable of secreting substances that contribute to the proliferation of their precursors and their differentiation into mature cells.

Basophils

Representing less than 1% of white blood cells, basophils have functions related to inflammatory processes.

Like many neutrophils and eosinophils, basophils are globular cells in suspension (10μm in diameter), but when they migrate into connective tissue they can have different shapes (pleomorphic).

Its nucleus has a characteristic “S” shape and large granules, a small Golgi complex, few mitochondria, and a large rough endoplasmic reticulum are found in the cytoplasm.

The small, specific granules of basophils are loaded with heparin, histamine, chemotactic factors, and peroxidases important for cell function.

Agranulocytes

Monocytes / macrophages

Monocytes represent about 8% of the total percentage of leukocytes in the body. They remain in circulation for a few days and differentiate into macrophages when they migrate into connective tissues. They are part of the responses of the specific immune system.

They are large cells, approximately 15μm in diameter. They have a large kidney-shaped nucleus that has a grainy appearance. Its cytoplasm is blue-gray in color, full of lysosomes and vacuole-like structures, glycogen granules, and some mitochondria.

Their main function is to engulf unwanted particles, but they also participate in the secretion of cytokines that are necessary for inflammatory and immunological reactions (as some are known as antigen-presenting cells).

These cells belong to the mononuclear phagocytic system, which is responsible for the "purification" or "cleaning" of dead cells or in apoptosis.

Lymphocytes

They are an abundant population of leukocytes (they represent about 25%). They are formed in the bone marrow and participate mainly in the reactions of the immune system, so their function is not exerted directly in the bloodstream, which they use as a means of transport.

Similar in size to erythrocytes, lymphocytes have a large and dense nucleus that occupies an important part of the cell. In general, all have little cytoplasm, few mitochondria, and a small Golgi complex associated with a reduced rough endoplasmic reticulum.

It is not possible to distinguish some lymphocytes from others by observing their morphological characteristics, but at an immunohistochemical level, thanks to the presence or absence of certain surface markers.

After their formation in the bone marrow, the maturation of these cells involves immune competition. Once they are immunologically competent, they travel to the lymphatic system and multiply there by mitosis, producing large populations of clonal cells capable of recognizing the same antigen.

Like monocytes / macrophages, lymphocytes are part of the specific immune system for the body's defense.

T lymphocytes

T lymphocytes are produced in the bone marrow, but they differentiate and acquire their immune capacity in the cortex of the thymus.

These cells are in charge of the cellular immune response and some can differentiate into cytotoxic or killer T cells, capable of degrading other foreign or deficient cells. They also participate in the initiation and development of the humoral immune reaction.

B lymphocytes

These lymphocytes, unlike T cells, are formed in the bone marrow and there they become immunologically competent.

They participate in the humoral immune response; that is, they differentiate as cells resident in plasma that are capable of recognizing antigens and producing antibodies against them.

Megakaryocytes

Megakaryocytes are cells larger than 50 µm in diameter with a large lobed polyploid nucleus and a cytoplasm filled with small granules with diffuse borders. They have an abundant rough endoplasmic reticulum and a well-developed Golgi complex.

They exist only in the bone marrow and are the progenitor cells of thrombocytes or platelets.

Platelets

Rather, these cells can be described as "cell fragments" originating from megakaryocytes, are disc-shaped and lack a nucleus. Its main function is to adhere to the endothelial lining of blood vessels to prevent bleeding in the event of injury.

Platelets are one of the smallest cells in the circulatory system. They are between 2 and 4μm in diameter and present two distinct regions (visible through electron micrographs) known as the hyalomer (a clear peripheral region) and the granulomer (a dark central region).

Mast cells

Mast cells or mast cells have their origin in the bone marrow, although their undifferentiated precursors are released into the blood. They have an important role in the development of allergies.

They have many cytoplasmic granules that house histamine and other "pharmacologically" active molecules that collaborate with their cellular functions.

References

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