Blood is a mixture of liquid and formed elements. The formed elements include the blood cells and the platelets, which are derived from stem cells in the bone marrow. Blood cells are divided into two groups: red blood cells (erythrocytes) and white blood cells (leukocytes). White blood cells are further categorized into specific types. Each type of blood cell has a different appearance that reflects its unique function in the body.
Red Blood Cells
Red blood cells (RBCs) are the most numerous of the circulating blood cells. Healthy adults have approximately 150 billion circulating RBCs per pound of body weight. RBCs make up roughly 40 percent to 50 percent of the whole blood volume in men, and 36 percent to 44 percent in women.
RBCs contain a substance called hemoglobin, which gives them their red color and enables them to perform their primary function: transporting oxygen from the lungs to the tissues of the body. RBCs also carry carbon dioxide from the tissues to the lungs, where it is exhaled.
Polymorphonuclear Leukocytes
Polymorphonuclear leukocytes (PMNs) are white blood cells. These blood cells are easily recognized under the microscope by their distinctive granules. Because of these granules, PMNs are also known as granulocytes. The granules of PMNs contain a variety of substances, which are released when PMNs attack an invading organism or when an allergic reaction occurs. PMNs comprise approximately 60 percent to 70 percent of circulating white blood cells.
Three varieties of PMNs circulate in the blood: neutrophils, eosinophils and basophils. Neutrophils are the most abundant of the PMNs. The primary function of neutrophils is to help fight infections resulting from ingesting invading bacteria. When the bacterium has been ingested, the contents of the neutrophil's granules are released, killing the invading organism.
Eosinophils are typically present in low numbers in the circulating blood. They function primarily to fight off parasitic infections. They also have a role in allergic and inflammatory responses. Although eosinophils are able to ingest bacteria, they are not as effective at killing them as neutrophils are.
Basophils are the least abundant of the circulating PMNs. They have an abundance of histamine in their granules and are involved in both allergic and inflammatory responses. They also have a role in host defense and immune response.
Lymphocytes
Lymphocytes are another type of white blood cell. They normal comprise 25 percent to 33 percent of circulating leukocytes. Lymphocytes are part of the immune system. They are responsible for antibody production, immune recognition, cellular immune responses and a variety of other immune functions. Although they are indistinguishable by appearance, there are two varieties of lymphocytes--T-cells and B-cells, which have different immunologic functions.
Monocytes
Monocytes arise from the same cell line as granulocytes. They typically comprise three percent to eight percent of the blood leukocytes. Monocytes circulate in the blood for only a few days before migrating into the tissues of the body, where they are transformed into macrophages. Like neutophils, monocytes are capable of ingesting invading organisms; they also ingest cellular debris. Monocytes have a key role in inflammatory responses and tissue repair. In addition, they perform important immune system functions.
References
- "Hematology Clinical and Laboratory Practice"; Rodger L. Bick, M.D., Editor-in-Chief; 1993
- "Robbins Pathologic Basis of Disease"; Ramzi S. Cotran, M.D., Vinay Kumar, M.D., Stanley L. Robbins, M.D., Frederick J. Schoen, M.D., Ph.D., Editors; 1994
- Medline Plus: Hematocrit
- "Mucosal Immunology"; Emerging Functions of Basophils in Protective and Allergic Immune Responses; Caroline L. Sokol, M.D., Ruslan Medzhitov, Ph.D.; January 2010
