Page 166 - 84 human physiolofy part-1

Chapter 9

Leukocytes are are divided into six basic types eosinophils, basophils, neutrophils, monocytes,
lymphocytes, and dendritic cells.

One functional group of leukocytes is the phagocytes, WBC that engulf and ingest their targets by
phagocytosis. This group includes the neutrophils, macrophages, monocytes (which are macrophage
precursors), and eosinophils. A second functional group is the cytotoxic cells, so named because they
kill the cells they attack. This group includes eosinophils and some types of lymphocytes.

Lets take a closer look at the six basic types of leukocytes.

Eosinophils

Eosinophils fight parasites and contribute to allergic reactions. They are easily recognized by the
bright pink staining granules in their cytoplasm. Normally, there are only a few eosinophils found in
the peripheral circulatory. They account for only 1-3% of all leukocytes. The life span of a typical
eosinophil in the blood is about 6-12 hours. Eosinophils are known to attach to large parasites and
release substances from their granules that damage or kill the parasite. Because eosinophils kill
pathogens, they are classified as cytotoxic cells. Eosinophils also participate in allergic reactions, by
contributing to inflammation and tissue damage by releasing toxic enzymes.

Basophils

Basophils release histamine and other chemicals. Basophils are rare in circulation but are easily
recognized in a stained blood smear by the large, dark blue granules in their cytoplasm. They also
release mediators that contribute to inflammation. The granules contain histamine, heparin(an
anticoagulant), cytokines, and other chemicals involved in allergic and immune responses.

Neutrophils

Neutrophils "eat" bacteria and release cytokines. Neutrophils are the most abundant WBC, 50-70%
of the total. They are easily identified by a segmented neucleus. Neutrophils, like other leukocytes are
formed in the bone marrow. They are phagocytic cells that typically ingest and kill bacteria. Most
neutrophils remain in the blood but can leave the circulation if attracted to an extravascular site of
damage or infection. In addition to ingesting bacteria and foreign particles, neutrophils release a variety
of cytokines.

Monocytes

Monocytes are the precursor cells of tissue macrophages. Monocytes are not that common in the
blood 1-6% of WBC. Once out of the blood, monocytes enlarge and differentiate into macrophages.
Some tissue macrophages patrol the tissues, creeping along by amoeboid motion. Others find a location
and remain fixed in place. Macrophages are the primary scavengers within tissues. Macrophages also
remove larger particles, such as old RBC and dead neutrophils. Macrophages play an important role in
the development of acquired immunity. After they ingest and digest molecular or cellular antigens,
fragments of processed antigen are inserted into the macrophage membrane as part of surface protein
complexes.

Lymphocytes Lymphocytes are the key cells that mediate the acquired immune response of the
body. Only about 5% of lyphocytes are found in circulation. They constitute 20-30% of all WBC. Most

166 | Human Physiology