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phagocyteA white BLOODcell (LEUKOCYTE) that
consumes pathogens during an IMMUNE RESPONSE
called PHAGOCYTOSIS. Granulocytes, and in particu-
lar neutrophils, are the primary phagocytes in the
circulating blood. Macrophages are phagocytes
that reside primarily in the LY M P Hstructures 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
PATHOGENor other cellular particle. Phagocytosis is
a key defense mechanism of the body’s IMMUNE
RESPONSEand may take place in the BLOOD, prima-
rily 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 LY M P Hcarry 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

releases as metabolic waste. The primary blood-

based 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 tis-

sues. Specialized phagocytes in the LIVER, the

Kupffer cells, function in a somewhat cannibalistic

manner, cleansing expired granulocytes from the

circulating blood and recycling their molecu-

lar components into the bloodstream for other

uses.

See also MONONUCLEAR PHAGOCYTE SYSTEM.

phlebotomy The clinical term for puncturing a

VEINwith a sterile needle to withdraw BLOOD. Phle-

botomy may be diagnostic, such as when drawing

blood for diagnostic blood tests, or therapeutic,

such as a treatment for HEMOCHROMATOSIS. Phle-

botomy may be mildly uncomfortable, as the nee-

dle may sting as it penetrates the SKINand the

vein. The blood withdrawal itself is painless. The

risks of phlebotomy are minor for most people

and include mild bleeding, bruising, and discom-

fort 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 sub-

stances dissolved in it including electrolytes, hor-

mones, 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 fresh-

frozen form. Plasma derivative products extracted

from donated plasma include cryoprecipitated anti-

hemophilic factor (AHF), ALBUMIN, IMMUNOGLOBULIN,

and Rh immunoglobulin.

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