Monocites and Neutrophils

Monocytes
Monocytes leave the blood and become macrophages and dendritic cells.
This scanning electron micrograph (courtesy of Drs. Jan M. Orenstein and Emma Shelton) shows a single macrophage surrounded by several lymphocytes. 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. This photomicrograph shows a single neutrophil surrounded by red blood cells. 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. This is a never-ending task, even in healthy people: Our throat, nasal passages, and colon harbor vast numbers of bacteria. Most of these are commensals, and do us no harm. But that is because neutrophils keep them in check. However, heavy doses of radiation chemotherapy and many other forms of stress can reduce the numbers of neutrophils so that formerly harmless bacteria begin to proliferate. The resulting opportunistic infection can be life-threatening.

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
The number of basophils also increases during infection. Basophils leave the blood and accumulate at the site of infection or other inflammation. There they discharge the contents of their granules, releasing a variety of mediators such as: histamine serotonin prostaglandins and leukotrienes which increase the blood flow to the area and in other ways add to the inflammatory process. 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.

Platelets
Platelets are cell fragments produced from megakaryocytes. Blood normally contains 150,000–450,000 per microliter (µl) or cubic millimeter (mm3). This number is normally maintained by a homeostatic (negative-feedback) mechanism. If this value should drop much below 50,000/µl, there is a danger of uncontrolled bleeding because of the essential role that platelets have in blood clotting. Some causes: certain drugs and herbal remedies; autoimmunity.

When blood vessels are cut or damaged, the loss of blood from the system must be stopped before shock and possible death occur. This is accomplished by solidification of the blood, a process called coagulation or clotting. A blood clot consists of a plug of platelets enmeshed in a network of insoluble fibrin molecules.

White Blood Cells

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 (although they all look alike under the microscope), each with different functions to perform .

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.