Cells of the blood

Red blood cells

Erythrocytes, or red blood cells (RBCs), have predictable dimensions which makes them a good “histologic ruler”. They are circular with a diameter of 7.8 micrometers. Erythrocytes are shaped like biconcave discs (donut). At their perimeter they are 2.6 micrometers thick, at the center they are only 0.8 micrometers thick. This shape increases the cell's surface area to allow for more oxygen binding. Because these dimensions are typically consistent, leukocytes can be identified by comparing their size to that of the erythrocytes.

The shape of erythrocytes depends largely on membrane proteins. Integral and peripheral membrane proteins maintain the biconcave shape of erythrocytes. There are more integral membrane proteins in the lipid bilayer than peripheral membrane proteins. They are classified as either glycophorin or band 3 proteins. Band 3 proteins bind to hemoglobin and act as an anchor for cytoskeletal proteins. Glycophorin proteins form the main attachments between the lipid bilayer and the cytoskeleton.

Hemoglobin

Hemoglobin is a globular protein found in erythrocytes. It is capable of binding and carrying several gases, including the vasodilator nitrous oxide, but its main function is to transport oxygen and carbon dioxide. Blood carries oxygen away from the lungs to the periphery and carbon dioxide from peripheral tissues back to the lung to facilitate gas exchange in the alveoli. A hemoglobin protein is made up of four polypeptide chains; two alpha chains and two beta chains, each featuring a heme group capable of binding oxygen. The heme group itself is shaped like a ring with an iron atom in the center.

Oxygen binds to hemoglobin through positive cooperation. That is, once an oxygen ion binds to a heme group, the iron atom is shifted in the ring which facilitates the binding of oxygen ions to the remaining three heme groups. The erythrocytes can then transport oxygen from the lungs to tissues throughout the body. There are approximately 250 million hemoglobin proteins in each erythrocyte therefore one red blood cell can carry around one billion oxygen ions at once.

Neutrophils

Neutrophils make up over half of the volume of white blood cells. The cytoplasm stains lightly and contains small, lavender colored granules. They are about 12-15 micrometers in diameter and have dark-staining multilobed nuclei. There are three types of granules in the cytoplasm of neutrophils: specific or secondary granules, azurophilic granules, and tertiary granules.

Specific granules are the smallest and most numerous and contain several enzymes: phospholipase and type IV collagenase. Azurophilic granules are the neutrophils’ lysosomes. There are two types of tertiary granules that contain either phosphatases or metalloproteinases. Neutrophils are active phagocytes that engulf bacteria. They can be considered the first responders of the (innate) immune system as they are often the first leukocytes to be activated in response to a pathogen.

Lymphocytes

The large, rounded nucleus of a lymphocyte takes up most of the volume of the cell and stains very dark blue. The cytoplasm appears as a narrow, lightly stained rim around the large nucleus where it does not contain granules.

Lymphocytes, the main functional cells of the immune system, are further classified by function and origin. For example, NK (natural killer) cells destroy virus infected and cancerous cells, B lymphocytes are involved in the production of circulating antibodies, and T lymphocytes are involved in cell-mediated immunity. Immunohistochemistry is required to identify individual types of lymphocytes in microscopy.

Eosinophils

Eosinophils contain many brightly staining granules in their cytoplasm that give them a characteristic pink color when viewed with a Wright’s stain. Their bilobed nuclei stain lightly in comparison to the granules. Eosinophils are also about 12-15 micrometers in diameter. Eosinophils release histaminase during  allergic reactions and are associated with inflammatory responses and helminthic responses to parasitic and protozoan infections.

Basophils

Basophils are the least abundant leukocytes in the blood; however, they are the largest granulocyte. The cytoplasm of basophils contains many blue granules that obscure the lightly stained bilobed nucleus. Basophils are associated with allergic responses as well: they release histamine and vasoactive agents to dilate blood vessels thereby intensifying the allergic reaction.

Monocytes

Monocytes are the largest leukocytes circulating in the blood at 12-20 micrometers in diameter. They are agranulocytes with a light and foamy cytoplasm. Monocytes are easily identified by their size and their large, kidney or horse-shoe shaped nucleus.

In the periphery, monocytes differentiate into phagocytic macrophages. Phagocytosis is a process in which a cell engulfs and ingest a macromolecule or microorganism. The macrophages differentiate further upon reaching their target tissues. Kupffer cells are found in the liver. Alveolar macrophages are found in the lung. The red pulp of the spleen contains splenic macrophages. Peritoneal macrophages are found free-floating in peritoneal fluid. Microglial cells are specialized macrophages found in the nervous system.