The immune cells we see in the peripheral blood are generally referred to as the white blood cells and include the cells of myeloid origin: neutrophils, basophils, eosinophils, and monocytes, and those of lymphoid origin: lymphocytes (Table 1). Although there are many types of lymphocytes, they cannot be differentiated visually when doing a blood smear analysis. All these cells exit ("extravasate") the vessels and enter various tissues to perform their function or enter the tissues to become activated to further differentiate or to further mature. For instance, the lymphocytes will migrate to the lymph nodes. They become activated by a specific antigen, divide and clone themselves, and further differentiate into effector or memory cells. In the process, the effector lymphocyte could further differentiate and mature based on their immediate cytokine environment. For instance, a CD4 T Helper lymphocyte could differentiate into a T Helper 2 (Th-2) cell or a T Helper 1 (Th-1) cell.
Table 1. Cells in Peripheral Blood.| Cell | Function |
| Neutrophils | Phagocytosis and killing of microorganisms; the first cells to migrate to the site of infection.
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| Lymphocytes | Many types and roles, but cannot be distinguished morphologically from each other. B Cells secrete antibodies, CD4 T Cells help in the immune response, CD8 T Cells and NK Cells directly kill infected cells.
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| Monocytes | Phagocytosis; circulating precursor cell to macrophage
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| Eosinophils | Respond to allergens and parasites
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| Basophils | Killing of parasites through the release of granules
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The first image shows two granulocytes - an eosinophil and a neutrophil, which are of myeloid origin. The second image is of a lymphocyte of lymphoid origin. These cells are commonly seen in a blood smear but must extravasate into the tissues to perform their functions.
