The buffy coat is a small but significant fraction of a whole blood sample that forms a thin, pale layer when the blood is separated. This layer is situated between the lighter blood plasma, which floats at the top, and the heavier red blood cells that settle at the bottom. The formation of the buffy coat is a direct result of the varying densities of the blood’s components. The concentration of specific cellular components in this layer makes it an extremely valuable resource for medical diagnostics and biological research.
How the Buffy Coat is Separated
The separation of the buffy coat from whole blood is achieved through a mechanical process known as centrifugation. Centrifugation involves spinning the blood sample at a high speed, which subjects the components to intense centrifugal force. This force effectively exaggerates the minute differences in density among the blood’s constituents, causing them to stratify into distinct layers.
The heaviest components, the red blood cells, are forced to the bottom of the collection tube or bag, forming the thickest layer. Conversely, the least dense component, the liquid plasma, remains at the top. The cellular elements that form the buffy coat, which are less dense than the red blood cells but heavier than the plasma, accumulate in a narrow band between the two main layers.
This thin, intermediate layer typically accounts for less than one percent of the total volume of the blood sample. Visually, the buffy coat presents as a pale, yellowish-white, or buff-colored band, which is where it derives its name. The successful separation of this layer allows laboratory technicians to isolate and collect the specific cells it contains without significant contamination from the surrounding plasma or red blood cells.
The Cellular Contents of the Buffy Coat
The buffy coat is primarily a concentrate of two cellular components: white blood cells (leukocytes) and platelets (thrombocytes). These cells are concentrated here because their specific gravities fall within a range intermediate to the plasma and the red blood cells. Although they constitute a small percentage of the total blood volume, they are highly concentrated within the buffy coat, often at a level 10 to 20 times greater than in whole blood.
The white blood cells found here, such as lymphocytes, monocytes, and granulocytes, are the body’s primary agents of immune defense. Lymphocytes and monocytes play roles in targeted immune responses, while granulocytes fight infections by releasing toxic granules. Platelets, which are small cell fragments, are also highly concentrated due to their similar density. These fragments are responsible for initiating blood clotting (hemostasis) by migrating to sites of vascular injury.
Why the Buffy Coat is Important
Isolating the buffy coat provides a concentrated source of specific cells, which is useful for various clinical and research applications. The high density of white blood cells makes the buffy coat an ideal starting material for the extraction of genomic DNA and RNA. Since mature red blood cells lack a nucleus, the nucleated white blood cells are the main source of genetic material, making the buffy coat a purer source for genetic studies.
In diagnostics, the buffy coat is used to test for the presence of infectious diseases, such as malaria, where the parasite may be more readily visible in the concentrated white blood cell layer. The white blood cells are also frequently isolated for immunological studies and cellular therapies. This includes preparing peripheral blood mononuclear cells for advanced research into diseases and the development of new treatments.
In blood banking, the buffy coat is a byproduct of separating whole blood into its components for transfusion purposes. The platelets isolated from multiple buffy coats can be pooled to create a therapeutic dose for patients with clotting disorders or low platelet counts, such as those undergoing chemotherapy.