Peripheral blood transports nutrients to all organs and systems of the body. Peripheral blood also plays an important role in excretion, carrying cellular waste products from the cells to the excretory system. In addition, the peripheral blood is an important part of the body's overall immunity, as the blood stream can eliminate or prevent pathogens from taking hold in different areas of the body.

Peripheral blood cells are the main constituents of the blood. They include red blood cells or "erythrocytes," white blood cells or leukocytes and platelets. 

Peripheral Blood Mononuclear Cells are commonly called PBMC. This type of cell groups together all the peripheral blood cells with a round nucleus. PBMCs include lymphocytes (B cells, T cells and NK cells) and monocytes. Erythrocytes and platelets do not have a nucleus and granulocytes (neutrophils, basophils and eosinophils) have a nucleus with several lobes. In humans, lymphocytes make up the majority of the PBMC population.