Blood Cells: Types, Classification, Normal Values, Functions

Almost every patient who has blood drawn gets a complete blood count, or CBC. That single report is a census of the cells in this article: how many red cells are carrying oxygen, whether the white cell count is rising against an infection, and whether there are enough platelets to stop bleeding. Learning the blood cells is learning how to read that report.

Blood is a fluid connective tissue that transports oxygen and nutrients to the cells and carries away carbon dioxide and other waste products. It is composed of a clear, straw-colored watery fluid known as plasma in which different types of blood cells are suspended.

Blood cells are different types of cells found in the blood but formed in bone marrow by a process known as hematopoiesis. These constitute about 45% of the volume of blood.

Classification of blood cells

Blood contains three types of formed elements, all suspended in plasma:

1. Red blood cells (erythrocytes), which carry oxygen and carbon dioxide.
2. White blood cells (leukocytes), which defend the body. They are further divided into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes and monocytes).
3. Platelets (thrombocytes), which are cell fragments that stop bleeding by forming clots.

In short: red cells for transport, white cells for defense, platelets for clotting.

Total white cell count in a healthy adult is about 4,000 to 11,000/µL. Reference ranges vary slightly by laboratory.

Red blood cells (RBCs)

A red blood cell is about 7.5 micrometers (µm) in diameter, which is roughly 0.0075 mm, with a normal range of 7.2 to 7.5 µm. It is a biconcave disc, around 2.5 µm thick at the rim and about 1 µm at the center, and it has no nucleus. This small, flexible, biconcave shape gives the cell a large surface area for gas exchange and lets it bend to pass through the narrowest capillaries.

Red blood cells are red colored and non-nucleated elements in the blood. These cells are red due to the presence of coloring substance known as hemoglobin. The location of production of red blood cells are in bone marrow. The hormone erythropoietin controls the production of RBCs. The other terms for red blood cells are erythrocytes, red cells, red blood corpuscles, haematids, or erythroid cells.

Figure: Morphological structure of RBCs

Morphology (structure) of Red blood cells

• Shaped: Normally, disc-shaped and biconcave.
• Size: 7.5 µm in diameter.
• Thickness: At the periphery, it is thicker with 2.5µm, and it is thinner at the center at 1µm. This difference in thickness is because of the biconcave shape.
• Surface area: 120 square µ.
• Volume: 85-90 cubic u.
• Composition: They are non-nucleated and covered with a membrane composed of protein and lipids. DNA and other cell organelles such as mitochondria and Golgi body are also absent in RBC.

Normal value of red blood cells

• Average red blood cell count is 4.5-6 million per cubic millimeter of blood.
• In adults male: 5-6 million per cubic millimeter of blood.
• In adult females:4.2-5.5 million per cubic millimeter of blood.

Life span of red blood cells

• The average life span of RBCs is about 120 days.
• After the lifespan, the old RBCs breakdown or hemolyse, carried out by macrophages in the spleen, bone marrow, and liver.
• Iron released by hemolysis returns to bone marrow to form new hemoglobin molecules.

Functions of Red Blood Cells

1. Transport of oxygen from the lungs to the tissues: Hemoglobin in the RBCs binds with oxygen to form oxyhemoglobin. About 97% of oxygen transports in the blood in the form of oxyhemoglobin.
2. Transport of carbon dioxide from the tissues to the lungs: about 70% of carbon dioxide is carried as bicarbonate, about 20 to 23% binds hemoglobin to form carbaminohemoglobin, and the rest is dissolved in plasma.
3. Buffer function: Hemoglobin in red blood cells acts as a buffer by regulating hydrogen ion concentration and thereby plays a role in maintaining acid base balance.
4. Blood group determination: Red blood cells carry the blood group antigens like A antigen, B antigen, and Rh factor. This helps in the determination of blood groups and enables the prevention of reactions due to incompatible blood transfusion.

White Blood Cells

White blood cells are the colorless and nucleated type of blood cell made in bone marrow but found in blood and lymph tissue. It is also known as leukocytes. WBCs are an essential part of the body’s immune system.

Figure: Classification of White Blood Cell

Classification of white blood cells

Based on the presence or absence of granules in the cytoplasm, the WBCs are classified into two groups: Granulocytes and Agranulocytes.

1. Granulocytes: Granulocytes have visible granules in the cytoplasm when stained and viewed under a microscope. It is further classified into three types; Neutrophils, Eosinophils, and Basophils.
2. Agranulocytes: Agranulocytes does not have visible granules in the cytoplasm when stained and viewed under a microscope. It is further classified into Monocytes and Lymphocytes.

Lymphocytes are further subdivided into T-lymphocytes, B-lymphocytes and NK (Natural Killer) cells.

Morphology of white blood cells

1. Neutrophils: They are mostly circular, but their shape changes into ameboid form once activated. Their diameter is 12 to 15 µm. They have fine or small granules in the cytoplasm, which take acidic or basic stains. The nucleus is multilobed. The number of lobes varies. In the younger cells, the nucleus is not lobed, but the older one has 3-5 lobes.
2. Eosinophils: The diameter of cells varies between 12 to 17 µm (eosinophil is the largest granulocyte). They consist of larger granules in their cytoplasm, which stain red or pink with eosin. The nucleus is segmented or bilobed and spectacle shaped.
3. Basophils: The diameter of the cell varies between 12 to 15 µm. They also have larger granules in their cytoplasm, which stains purple-blue with methylene blue. The nucleus is bilobed and is ‘S-shaped.’
4. Monocytes: They are the largest leukocytes, with a diameter of 15 to 20 µm (largest WBC). They are motile and, on maturation, develop into macrophages and dendritic cells. The cytoplasm is clear without granules. The nucleus is round, oval, and horseshoe, bean, or kidney-shaped is placed at one side.
5. Lymphocytes: They have clear cytoplasm without granules. The nucleus is oval, bean-shaped, or kidney-shaped, occupying the whole of the cytoplasm. These are the only blood cells with specific receptors for antigens. Thus they are key mediators of adaptive immunity. Although all types of lymphocytes are morphologically similar and rather unremarkable in appearance, they are extremely heterogeneous in lineage, function, and phenotype and are capable of complex biologic responses and activities. So, the use of panels of monoclonal antibodies specific to the surface proteins helps differentiate these by the surface proteins. The standard nomenclature for these proteins is the “CD” (cluster of differentiation) numerical designation, which helps to delineate surface proteins that define a particular cell type or stage of cell differentiation.

Normal count of white blood cells

• Total white blood count (TLC or TC): 4,000-11,000/cumm of blood.
• Differential white blood count is given in the table below

Life span of white blood cells

The lifespan of WBC varies, depending upon the demand of the body and its function. Lifespan can be as short as a few hours or maybe can last as long as 3 to 6 months. However, the normal lifespan of WBCs is given in the table below

Functions of white blood cells

WBCs play a crucial role in the defense mechanism of the body. Each type of WBCs functions in different ways;

1. Neutrophils play a vital role in the defense mechanism of the body. Along with monocytes, it provides a first-line defense against invading microorganisms. Similarly, it also helps to boost the response of other cells.
2. Eosinophils play an essential role in the host’s defense against parasites.
3. Basophils play an essential role in allergy.
4. Monocytes help the body’s defense mechanism by a process known as phagocytosis.
5. Lymphocytes play a major role in immunity. B lymphocytes are the only cells capable of producing antibodies; therefore, they are the cells that mediate humoral immunity. T-Lymphocytes are the cells of cell-mediated immunity. The antigen receptors of T lymphocytes only recognize peptide fragments of protein antigens bound to major histocompatibility complex (MHC) molecules, on the surface antigen-presenting cells (APCs). Natural killer cells provide innate killing of virus-infected and tumor cells) so the page agrees with itself

Platelets

Platelets are small, colorless, non-nucleated, and moderately refractive bodies. They are produced when large cell known as megakaryocytes breaks into fragments of 2000-3000 in numbers.

Figure: Platelets in Giemsa Stained blood film

Morphology of platelets

• Diameter: 2-4 µ
• Volume: 7.5 cu µ
• Shape: They are of several shapes, spherical or rod-shaped, and become oval and disk-shaped when inactivated. Sometimes, the platelets are of dumbell shape, comma shape, cigar shape, or any other unusual form.

Lifespan of platelets

• The average lifespan of platelets is 7-10 days.
• The tissue macrophage system destroys platelets in the spleen. So, splenomegaly (enlargement of the spleen) decreases platelet count, and splenectomy (removal of the spleen) increases the platelet count.

Normal count of platelets: 150,000 to 450,000/µL, average about 250,000 /µL

Functions of platelets

1. It helps to prevent blood loss (hemostasis) in three ways;
2. Platelets secrete 5HT, which causes constriction of blood vessels. The adhesive property of platelets helps to seal damaged blood vessels such as; capillaries. Platelets help seal the damaged part of the blood vessel by forming a temporary plug.
3. The platelets are responsible for forming an intrinsic prothrombin activator responsible for the onset of blood clotting.
4. The platelets are useful for repairing the endothelium and other structure of blood vessels. They can interact with and help trap some microbes.
5. The cytoplasm of platelets contains contractile proteins, namely actin, myosin, and thrombosthenin. These contractile proteins are responsible for clot retraction.

Where students get confused

A normal differential percentage does not mean the counts are normal. The differential gives each white cell as a percentage of the total white cells. A percentage only means something next to the total count. A patient can show a normal-looking 60% neutrophils while the total white count is dangerously low. Always read the differential together with the total: the absolute count equals the percentage times the total count.

Serum and plasma are not the same, and blood cells travel in plasma. Blood cells are suspended in plasma, the straw-colored fluid that still contains clotting factors such as fibrinogen. Serum is what is left after blood has clotted, so it is plasma minus the clotting factors. In the body, cells move in plasma, not serum.

A "left shift" is not a position on the report. A left shift means the blood contains more young, immature neutrophils (band forms and earlier stages) that the bone marrow releases during acute infection. The name comes from old counting charts where immature cells were drawn on the left. It describes the cells, not a place on your lab sheet.

Anisocytosis and poikilocytosis are different. Anisocytosis is variation in red cell size (aniso means unequal). Poikilocytosis is variation in red cell shape (poikilo means varied). One is about size, the other about shape.

Carbaminohemoglobin is not carboxyhemoglobin. Carbaminohemoglobin is hemoglobin carrying carbon dioxide, part of normal CO2 transport. Carboxyhemoglobin is hemoglobin bound to carbon monoxide, which is carbon monoxide poisoning. Swapping these two is a common exam mistake.

The most abundant blood cell and the most abundant white cell are not the same. Red blood cells are by far the most abundant cells in blood overall. Among white cells only, neutrophils are the most abundant. Keep the two rankings separate.

References

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3. Barrett KE, Barman SM, Brooks HL, Yuan JX. Ganong's Review of Medical Physiology. 26th ed. McGraw Hill; 2019.
4. Bain BJ, Bates I, Laffan MA. Dacie and Lewis Practical Haematology. 12th ed. Elsevier; 2017.
5. Hoffbrand AV, Moss PAH. Hoffbrand's Essential Haematology. 8th ed. Wiley-Blackwell; 2019.
6. Delves PJ, Martin SJ, Burton DR, Roitt IM. Roitt's Essential Immunology. 13th ed. Wiley-Blackwell; 2017.
7. Patel AA, Ginhoux F, Yona S. Monocytes, macrophages, dendritic cells and neutrophils: an update on lifespan kinetics in health and disease. Immunology. 2021;163(3):250-261. https://doi.org/10.1111/imm.13320