Contents:
White Blood Cells
- White blood cells are important components of the blood.
- They are found in the blood and lymphatic tissue.
- On centrifugation of a blood sample, a thin layer appears between the RBCs and the blood plasma.
- This layer is called a buffy coat and it contains the nucleated WBCs.
- The normal count of WBCs is 5000-10000 cells per mm3 of blood
- White blood cells or leukocytes have nuclei and do not contain hemoglobin.
- WBCs are classified as either granular or agranular, depending on the basis of the presence or absence of granules.
Granulocytes
- About 75% of total WBC are granulocytes.
- These cells have differently shaped nuclei called polymorphonuclear leukocytes (PMNs)
or polymorphs. - When observed under the microscope these cells show the presence of granules in the
cytoplasm hence, they are called as granulocytes. - The cells are divided into three types depending on the staining characteristics of
granules.
(1) Eosinophils or acidophils: The granules of cell when stained with acidic dyes shows red-orange color hence called acidophils or eosinophils.
(2) Neutrophils: The granules of cells do not show staining with dye so-called as neutrophils.
(3) Basophils: The granules of cells when stained with basic dyes shows blue-purple color so-called basophils.
Neutrophils:
- They are about 60-70 % of total WBCs.
- Life span: One or two days
- Average diameter: 10 to 12 um
- Nucleus: 1 to 4 lobed
Functions:
- They reach the area of infection and actively phagocytize the micro-organism.
- Neutrophils also contain defensin protein which exhibits a broad range of antibiotic activity
- against bacteria and fungi.
Eosinophil:
- They are about 2-4% of total WBCs.
- Life span: One to two days
- Average diameter: 10 to 12 um
- Nucleus: Two lobes, the cytoplasm contains large granules
- Staining: Granules stained by red acidic dyes
Functions:
- They leave the capillaries and enter the tissue fluid where they release enzymes like histaminases, that combat the effect of histamine and other substances involved in inflammation during an allergic reaction.
- They phagocytize Ag-Ab complexes and are effective against certain parasitic worms.
- A high eosinophil count generally indicates an allergic condition or parasitic infection.
Basophils:
- They are about 0.5- 1% of total WBCs.
- Life span: One to two days
- Average diameter: 8 to 10 um
- Nucleus: Irregular and usually bilobed, cytoplasm contains large granule
- Staining: Granules stained with basic, purple-blue color dyes.
Functions:
- At the site of inflammation, basophils leave the capillaries, enter tissues and release granules that contain heparin, histamine, proteases, and serotonin.
- These substances intensify the inflammatory reaction and are involved in hypersensitivity (allergic) reactions.
Agranulocytes
- These are characterized by the absence of distinct granules in their cytoplasm.
- Agranular leucocyte possesses cytoplasmic granules, which are not visible under the light microscope because of their small size and poor staining qualities.
- There are two types of agranular leukocytes:
- Lymphocytes
- Monocytes
Lymphocytes:
- The classification of lymphocytes is small and large lymphocytes.
- Small lymphocytes: 6-9 um
- Large lymphocytes: 10-14 um
- Nucleus: Round
- Percentage: 20 to 25% of total WBCs
- Depending on their site of production and their action lymphocytes are divided into:
B cells:
- These are developed from the bone marrow stem cells.
- B cells are critical to humoral immunity as this type of immunity relies on the circulation of antibodies in the bodily fluids and blood serum to identify and counteract antigens.
- B-lymphocytes are converted to plasma cells which secrets antibodies.
T cells:
- These are developed from the thymus.
- T cells are responsible for cell-mediated immunity.
- There are three major classes of T cells that play specific roles-cytotoxic-T cells, helper T cells, and regulatory cells.
- Cytotoxic-T cells directly eliminate cells containing antigens by binding to them and lysing or causing them to burst open.
- Helper-T cells participate in the production of antibodies by B cells and also produce substances that activate other T cells.
- Regulatory-T cells (Suppressor-T cells) suppress the response of B cells and other T cells to antigens.
Natural killer (NK) cells:
- These cells attack a wide variety of infectious microbes.
- The NK cells are non-specific in action.
- The NK cells release chemicals within their granules to break down the cell membrane of diseases or tumor cells.
- NK cells can also induce virus-infected cells and cancer cells to undergo apoptosis.
Monocytes:
- They are 3 to 8% of total WBCs.
- Diameter: 12-20 um
- Nucleus: Oval or kidney-shaped
- The monocyte migrates into the tissues, enlarges, and differentiates into macrophages.
- There are two types of macrophages:
- Fixed macrophages: This resides in a particular tissue. E.g. alveolar macrophages in the lungs.
- Wandering macrophages: These macrophages migrate from one tissue to another and gather at the site of infection.
- Functions: They act as critical immune effector cells, phagocytose foreign substances (after transferring into fixed or wandering macrophages) and contribute to wound healing.
Platelets or Thrombocytes:
- They are the most numerous of all blood cells.
- The normal platelet count is 150,000 to 400,000 platelets in each pl of blood.
- Shape: Disc-shaped
- Diameter: 2 to 4 um
- Normal life span: 5 to 9 days
- Platelets have many vesicles but no nucleus.
- Non-nucleated cells containing granules in the cytoplasm.
Functions:
- It plays a major role in hemostasis.
- They prevent blood loss from damaged blood vessels by forming a platelet plug.
You May Read: Red Blood Cells: Structure, Function and Life Cycle