White Blood Cells (leukocytes)

White Blood Cells (leukocytes)

White blood cells


are much less numerous than red (the ratio between the two is around 1:700),
have nuclei,
participate in protecting the body from infection,
consist of lymphocytes and monocytes with relatively clear cytoplasm, and three types of granulocytes, whose cytoplasm is filled with granules.


Lymphocytes: There are several kinds of lymphocytes, each with different functions to perform , 25% of wbc The most common types of lymphocytes are


B lymphocytes ("B cells"). These are responsible for making antibodies.
T lymphocytes ("T cells"). There are several subsets of these:

inflammatory T cells that recruit macrophages and neutrophils to the site of infection or other tissue damage
cytotoxic T lymphocytes (CTLs) that kill virus-infected and, perhaps, tumor cells
helper T cells that enhance the production of antibodies by B cells




Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T cells migrate from the bone marrow to the thymus where they mature. Both B cells and T cells also take up residence in lymph nodes, the spleen and other tissues where they


encounter antigens;
continue to divide by mitosis;
mature into fully functional cells.


Monocytes : also originate in marrow, spend up to 20 days in the circulation, then travel to the tissues where they become macrophages. Macrophages are the most important phagocyte outside the circulation. Monocytes are about 9% of normal wbc count

Macrophages are large, phagocytic cells that engulf


foreign material (antigens) that enter the body
dead and dying cells of the body.


Neutrophils

The most abundant of the WBCs. about 65% of normal white count  These cells spend 8 to 10 days in the circulation making their way to sites of infection etc  Neutrophils squeeze through the capillary walls and into infected tissue where they kill the invaders (e.g., bacteria) and then engulf the remnants by phagocytosis. They have two types of granules: the most numerous are specific granules which contain bactericidal agents such as lysozyme; the azurophilic granules are lysosomes containing peroxidase and other enzymes

Eosinophils : The number of eosinophils in the blood is normally quite low (0–450/µl). However, their numbers increase sharply in certain diseases, especially infections by parasitic worms. Eosinophils are cytotoxic, releasing the contents of their granules on the invader.

Basophils : rare except during infections where these cells mediate inflammation by secreting histamine and heparan sulfate (related to the anticoagulant heparin). Histamine makes blood vessels permeable and heparin inhibits blood clotting. Basophils are functionally related to mast cells.  . The mediators released by basophils also play an important part in some allergic responses such as hay fever and an anaphylactic response to insect stings.

Thrombocytes (platelets):

Thrombocytes are cellular derivatives from megakaryocytes which contain factors responsible for the intrinsic clotting mechanism. They represent fragmented cells  which contain residual organelles including rough endoplasmic reticulum and Golgi apparati. They are only 2-microns in diameter, are seen in peripheral blood either singly or, often, in clusters, and have a lifespan of 10 days.