There are three categories of blood cells:
The erythrocytes comprise the largest population of blood cells with approximately 4.5-5.5 million RBCs per µL blood. The red blood cells normally account for about 40-45% of the total volume of a blood sample. This percentage, known as the hematocrit, is quite variable, depending upon a person's age, gender and other factors. The hematocrit is an integral part of the complete blood count.
The thrombocytes make up the second largest population of blood cells with normally 150,000-400,000 cells per µL blood, while only about 4-11 leukocytes occur per µL blood (4,000-11,000 per mL blood).
Only the leukocytes are true, complete cells, being the only mature blood cells that contain a nucleus and organelles. The erythrocytes and platelets function as membranous containers for certain proteins and molecules (e.g. hemoglobin within RBCs, or within platelets components important for coagulation) that normally should not occur freely within the blood plasma.
Following their synthesis in the bone marrow red blood cells and platelets enter the bloodstream where, under physiological conditions, they remain for the rest of their life cycle. Unlike RBCs and platelets, white blood cells are motile and are found not only in the bloodstream but throughout the body.
Although some white blood cells can live for years, most blood cells live for only a matter of days or weeks.
White blood cells are the cells of the immune system and are involved in defending the human body against infectious diseases, abnormal and cancerous cells, and foreign invaders (e.g. parasites or mycoses).
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