"Erythrocytes (red blood cells) are present at 4.4–6.0 million per microliter. They are flattened biconcave disks with no nucleus and are pale red. They transport oxygen and some carbon dioxide between tissue and lungs and have a lifespan of approximately 120 days. Leukocytes (white blood cells) number approximately 7000 per microliter (range 5000–10,000). They have an obvious dark-staining nucleus, function in body defenses, exit capillaries and move into tissues, and usually live for a few hours or days. Granulocytes, including neutrophils, eosinophils, and basophils, number approximately 4360 per microliter (range 1800–9950). They have abundant granules in the cytoplasm and a normally lobed nucleus. They provide nonspecific, or innate, resistance to disease and are classified according to membrane-bound granules in the cytoplasm. Agranulocytes, including lymphocytes and monocytes, number approximately 2640 per microliter (range 1700–4950). They lack abundant granules in the cytoplasm and have a simple-shaped nucleus that may be indented. They function in body defenses, and the group consists of two major cell types from different lineages. Lymphocytes make up 20–40% of total leukocytes and number approximately 2185 per microliter (range 1500–4000). They are spherical cells with a single, often large, nucleus occupying much of the cell’s volume; the nucleus stains purple. They occur in large natural killer cell and small B-cell and T-cell variants. Lymphocytes provide primarily specific, or adaptive, immunity: T cells directly attack other cells in cellular immunity, B cells release antibodies in humoral immunity, and natural killer cells are similar to T cells but nonspecific. Initial cells originate in bone marrow, but secondary production occurs in lymphatic tissue. Several distinct subtypes exist, and memory cells form after exposure to a pathogen and rapidly increase in response to subsequent exposure. Their lifespan may be many years. Monocytes make up 1–6% of total leukocytes and number approximately 455 per microliter (range 200–950). They are the largest leukocytes and have an indented or horseshoe-shaped nucleus. They are very effective phagocytic cells that engulf pathogens or worn-out cells and also serve as antigen-presenting cells or other components of the immune system. They are produced in red bone marrow and are referred to as macrophages and dendritic cells after leaving the circulation. Platelets number approximately 350,000 per microliter (range 150,000–500,000). They are cellular fragments surrounded by a plasma membrane and containing granules that stain purple. They function in hemostasis and release growth factors for repair and healing of tissue. They are formed from megakaryocytes that remain in the red bone marrow and shed platelets into circulation. Formed elements of blood include erythrocytes, leukocytes, and platelets."