Leucocytes (white blood cells) defend the body against infection by pathogens and help to remove damaged cells and debris.Compared with erythrocytes, leucocytes:• always have a nucleus, often large or lobed, which makes them easy to identify on a stained blood smear
• are usually larger
• are much less numerous: about 4000 to 11 000 per mm3, roughly one for every 500 to 1000 red cells
• do not contain haemoglobin, so they appear colourless before staining
• contain many mitochondria and lysosomesLeucocytes are made from stem cells in the bone marrow. Some types mature in other organs: for example, T lymphocytes mature in the thymus.Many leucocytes spend only a short time in the blood. They can change shape and squeeze out between the cells of capillary walls into the tissues, especially at sites of infection. They are attracted there by chemicals released by damaged cells and pathogens. Because of this, the number of leucocytes in a blood sample rises during an infection, and a white cell count is a useful medical test.Leucocytes defend the body in two main ways:• phagocytosis: engulfing and digesting pathogens
• the specific immune response: producing antibodies and killing infected cellsLeucocytes are classified into two groups by the appearance of their cytoplasm and nucleus when stained: granulocytes, which have granules in their cytoplasm and a lobed nucleus, and agranulocytes, which have clear cytoplasm and an unlobed nucleus. These groups are covered in the next two lessons.
• are usually larger
• are much less numerous: about 4000 to 11 000 per mm3, roughly one for every 500 to 1000 red cells
• do not contain haemoglobin, so they appear colourless before staining
• contain many mitochondria and lysosomesLeucocytes are made from stem cells in the bone marrow. Some types mature in other organs: for example, T lymphocytes mature in the thymus.Many leucocytes spend only a short time in the blood. They can change shape and squeeze out between the cells of capillary walls into the tissues, especially at sites of infection. They are attracted there by chemicals released by damaged cells and pathogens. Because of this, the number of leucocytes in a blood sample rises during an infection, and a white cell count is a useful medical test.Leucocytes defend the body in two main ways:• phagocytosis: engulfing and digesting pathogens
• the specific immune response: producing antibodies and killing infected cellsLeucocytes are classified into two groups by the appearance of their cytoplasm and nucleus when stained: granulocytes, which have granules in their cytoplasm and a lobed nucleus, and agranulocytes, which have clear cytoplasm and an unlobed nucleus. These groups are covered in the next two lessons.