Origin of Peripheral Blood Mononuclear Cells

Peripheral blood mononuclear cells (PBMCs) originate from hematopoietic stem cells (HSCs) located in the bone marrow. HSCs are responsible for producing all blood cells and the immune system through a process known as hematopoiesis. These stem cells differentiate into myeloid cells (monocytes, macrophages, granulocytes, megakaryocytes, dendritic cells, red blood cells) and lymphoid cells (T cells, B cells, natural killer (NK) cells).

Within these two lineages, the cells that comprise PBMCs are found. PBMCs are nucleated blood cells that form a heterogeneous population including various lymphocyte populations (T cells, B cells, NK cells), dendritic cells, and monocytes. These cells are vital components of both the innate and adaptive immune systems, protecting the body against viral, bacterial, and parasitic infections, as well as eliminating tumor cells and foreign materials.

Distribution of Various Cell Populations in PBMCs

The composition of human cell populations varies among individuals; however, on average, lymphocytes constitute the majority of PBMCs. Lymphocytes play a crucial role in cellular and humoral immune responses, primarily associated with the activation of T and B cells.

Among the lymphocyte population, CD3+ T cells represent the largest cellular component (45-70%). Most T cells exist as naïve and resting cells, which are not activated by antigens or function as memory T cells. The activation of naïve T cells occurs through antigen recognition, forming a small population of T cells in healthy individuals. Upon activation, T cells initiate a cellular immune response targeting antigens present in infected or diseased cells.

Similarly, CD19+ B cells exist as naïve or memory cells awaiting activation by antigens, comprising only 5-15% of the total lymphocyte population. After activation, B cells differentiate into plasma cells capable of secreting antibodies that specifically target free antigens circulating in the bloodstream. The ability of secreted antibodies to target free antigens in the extracellular space is defined as the humoral immune response.

NK cells constitute a smaller fraction of the lymphocyte population (5-10%). These cells perform their functions effectively without the need for prior antigen exposure and defend the body against tumor activity.

A small proportion of white blood cells includes dendritic cells (1-2%), which form a crucial interface between the innate and adaptive immune systems. Dendritic cells, as highly specialized antigen-presenting cells, engulf antigens and present antigenic fragments to adaptive immune cells, facilitating the activation of T and B cells.

Monocytes differ in complexity and size from lymphocytes, comprising 10-30% of the population. Monocytes circulate in the bloodstream and peripheral tissues, and upon stimulation, they mature into dendritic cells or macrophages, contributing to the functions of both innate and adaptive immune responses, acting as phagocytic cells and antigen presenters.

Hematopoietic stem cells are of paramount importance. HSCs in the blood and bone marrow produce all blood cells, including red blood cells, platelets, lymphocytes, monocytes, and granulocytes. For stem cell transplantation, this population is rare, constituting only 0.1-0.2% of PBMCs, making their isolation from whole blood samples challenging.

Isolation of Peripheral Blood Mononuclear Cells

There are two main methods for isolating PBMCs from whole blood. These methods utilize centrifugation to create a density gradient or employ leukapheresis.

Since cells have specific densities, the density gradient centrifugation process separates major cell populations, such as lymphocytes, monocytes, granulocytes, and red blood cells, by creating a density gradient. For human cells, the medium density of 1.077 grams per milliliter allows for adequate separation of PBMCs (density of 1.077 grams per milliliter) from red blood cells and granulocytes (density > 1.077 grams per milliliter). After centrifugation, PBMCs appear as a thin white layer between the plasma and the density gradient medium, facilitating their isolation.

The leukapheresis device is an automated instrument that utilizes high-speed centrifugation to separate PBMCs from whole blood.