Leucocytes | White Blood Corpuscles | WBCs

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Leucocytes

• Leucocytes do not have haemoglobin.

• Number. The number of leucocytes per microlitre of blood is called the total leucocyte count (TLC).
• This varies from 6000 to 8000 mm of blood. Thus , they are less in number than the RBCs.
• The ratio of WBC : RBC in our blood is 1 : 700.
• Rise in WBC count is termed leucocytosis.
• Increased TLC shows that there is acute bacterial infection.
• Abnormal increase of WBCs is in malignancies like leukemia (blood cancer).
• Fall in WBC count is called leukopenia.
• In some conditions, such as folic acid deficiency, the total count of WBC decreases.
• The total count of WBC is useful in diagnosing various diseases.
• Normal or low TLC is in viral infection, malaria, typhoid or tuberculosis. The ratio of leucocyte to ery throcyte is 1.600

• Shape. The leucocytes are rounded or irregular in shape.
• They can change their shape like Amoeba and are thus, capable of amoeboid movement.
• This enables them to squeeze out of blood capillaries into the tissues (extra vascular regions).
• This process is called diapedesis.

• Size. The WBCs are larger than the RBCs. Their size is from 12 to 20 pm.

• Colour. The WBCs are colourless.

• Structure. A leucocyte consists of cell membrane, nucleus and cytoplasm. The cytoplasm contains mitochondria, Golgi apparatus, centrioles besides other cell organelles.

• Types. The leucocytes are of two main types : Agranulocytes and granulocytes.

Agranulocytes

• The granules are not found in the cytoplasm of these cells.
• The agranulocytes are of two types .
• Lymphocytes. They are smaller in size containing scant cytoplasm with large rounded nucleus.
• They are nonmotile and nonphagocytic.
• They produce antibodies to de stroy microbes and their toxins reject grafts and kill tumour cells.
• They also help in healing of injuries.
• Lymphocytes exist in two major groups in circulation.
• These are B- and T lymphocytes. Lymphocytes are 20-25 % of leucocytes
• Monocytes. They are the largest of all types of leucocytes and somewhat amoeboid in shape.
• They have much cytoplasm. The nucleus is bean - shaped.
• They are motile and phagocytic in nature and engulf bacteria and cellular debris . Generally they change into macrophages after entering tissue spaces.
• Monocytes are 2-10 % of of leucocytes.

Granulocytes

• They contain granules in their cytoplasm.
• Their nucleus is irregular or lobed or subdivided.
• According to their staining property, the granulocytes are divided into three types
• Eosinophils. The nucleus is two lobed. They have coarse granules.
• Their granules take acidic stains (e.g. , eosin).
• Their number increases in people with allergic conditions such as asthma or hay fever.
• They also help in dissolving blood clot. They are nonphagocytic.
• They seem to play a part in the immune system . They have some similarity to lysosomes.
• Eosinophils can attach themselves to parasitic forms and cause their destruction by liberating lysosomal enzymes on their surface.
• Eosinophils are 2-3 % of leucocytes.
• Basophils. The nucleus is usually three lobed.
• They have less number of coarse granules.
• Their granules take basic stain (e.g. , methylene blue) strongly.
• They release heparin, serotonin and histamine.
• They are probably like mast cells of connective tissue.
• Basophils are least (0-1 %) among them.
• Neutrophils. The nucleus is many lobed. They have fine granules.
• They stain weakly with both acid and basic stains.
• Neutrophils are the most numerous of all leucocytes.
• Certain neutrophils in female mammals possess a small spherical lobe attached to their nucleus by a stalk.
• This lobe is called drum stick (= sex chromatin) or Barr body.
• Barr is the name of the scientist.
• Drumstick is formed by transformation of an X - chromosome.
• They eat harmful germs and are, therefore, phagocytic in nature. Neutrophils are 60-65 % of leucocytes.

• Formation. Formation of leucocytes is called leucocytosis or leucopoeisis
• The granu locytes and monocytes are formed only in bone marrow.
• Lymphocytes are produced mainly in lymph nodes ; spleen, thymus, tonsils, bone marrow and Peyer's patches of small intestine.

• Life Span. The life of the granulocytes once released from the bone marrow is normally 4 to 8 hours circulating in the blood and another 4 to 5 days in the tissues.
• The monocytes also have a short life span of 10 to 20 hours.
• The lymphocytes have life spans of few days or months or even years, but this depends on the body's need for these cells.

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