Encyclopedia of Health and Medicine

P
phagocyte A white BLOOD cell (LEUKOCYTE) that
consumes pathogens during an IMMUNE RESPONSE
called PHAGOCYTOSIS. Granulocytes, and in particular
neutrophils, are the primary phagocytes in the
circulating blood. Macrophages are phagocytes
that reside primarily in the LYMPH structures and
the tissues. Protein markers on the surface of the
PATHOGEN, called opsonins, attract phagocytes to
the pathogen’s location. The phagocyte extends its
wall to encircle the pathogen, then releases
enzymes that digest the pathogen. An individual
phagocyte may digest up to a dozen pathogenic
particles during the course of its existence.
See also BACTERIA; CELL STRUCTURE AND FUNCTION;
MONONUCLEAR PHAGOCYTE SYSTEM.
phagocytosis The process through which a
PHAGOCYTE (a specialized LEUKOCYTE) consumes a
PATHOGEN or other cellular particle. Phagocytosis is
a key defense mechanism of the body’s IMMUNE
RESPONSE and may take place in the BLOOD, primarily
the domain of granulocytes (neutrophils and
eosinophils), and in the tissues, primarily the
realm of tissue-based monocytes (macrophages).
When a pathogen invades the body, the immune
response sends substances called opsonins to coat
its surface. This process, called opsonization,
marks the pathogen, attracting phagocytes. The
most common opsonins are antibodies and the
proteins the complement system produces.
The blood and the LYMPH carry phagocytes to
the sites of opsonized pathogens. When the
phagocyte reaches the pathogen it extends its cell
wall to enclose the pathogen within its cytoplasm.
Once enclosure is complete the phagocyte releases
enzymes called lysozymes that digest the
pathogen, breaking it down into its molecular
components which the cell then recycles or
162
releases as metabolic waste. The primary bloodbased
phagocytes are neutrophils, which respond
to pathogens, and eosinophils, which respond to
antigens. Monocytes circulate in the blood only
for about 12 hours and then migrate into the tissues.
Specialized phagocytes in the LIVER, the
Kupffer cells, function in a somewhat cannibalistic
manner, cleansing expired granulocytes from the
circulating blood and recycling their molecular
components into the bloodstream for other
uses.
See also MONONUCLEAR PHAGOCYTE SYSTEM.
phlebotomy The clinical term for puncturing a
VEIN with a sterile needle to withdraw BLOOD. Phlebotomy
may be diagnostic, such as when drawing
blood for diagnostic blood tests, or therapeutic,
such as a treatment for HEMOCHROMATOSIS. Phlebotomy
may be mildly uncomfortable, as the needle
may sting as it penetrates the SKIN and the
vein. The blood withdrawal itself is painless. The
risks of phlebotomy are minor for most people
and include mild bleeding, bruising, and discomfort
at the puncture site.
See also BLOOD DONATION.
plasma The liquid portion of the BLOOD. Plasma is
about 90 percent water and makes up 55 percent of
the total blood volume. It contains numerous substances
dissolved in it including electrolytes, hormones,
enzymes, antibodies, GLUCOSE, and CLOTTING
FACTORS (specialized proteins). It also carries the
blood cells in suspension. Plasma is available for
transfusion as a blood product, in fresh or freshfrozen
form. Plasma derivative products extracted
from donated plasma include cryoprecipitated antihemophilic
factor (AHF), ALBUMIN, IMMUNOGLOBULIN,
and Rh immunoglobulin.