Page 309 - Anatomy and Physiology of Farm Animals, 8th Edition

294 / Anatomy and Physiology of Farm Animals

Platelets reduce loss of blood from Chemical messengers known as chemo­
tactic factors attract neutrophils to these
injured vessels. By adhering to vessel walls
VetBooks.ir and to each other in the area of the injury, sites. Chemotactic factor is a general term
platelets may form a plug upon which a
for a variety of compounds that are capable
thrombus (clot) forms to occlude the of attracting neutrophils and in some
opening in the vessel and prevent further cases other leukocytes. These factors may
blood loss. Substances released by platelets be produced by invading microorganisms,
and lodged on their surface membranes by leukocytes after interacting with
stimulate clotting and help cause local microorganisms, or by damaged tissue.
constriction of the injured blood vessel Neutrophils are phagocytes; they engulf
(described in more detail later in this invading bacteria to destroy them. The
chapter). destruction involves the action of enzymes
in the intracellular granules of the neutro-
Leukocytes phils and cell membrane–bound enzymes
that are activated when phagocytic vesicles
are completely formed. During phagocytosis,
Leukocytes (see Fig. 1‐9) or white blood neutrophils may also release enzymes that
cells (from the Greek word leuco, white) contribute to the local inflammation. In
differ considerably from erythrocytes in the process of responding to the potential
that they are nucleated and are capable of infection, many of the attracted neutrophils
independent movement to exit blood ves- are destroyed. Purulent debris (sometimes
sels. Leukocytes may be classified as either called “pus”), is the semiliquid material
granulocytes or agranulocytes based on that results from the collective responses
the presence or absence of cytoplasmic to a microbial invasion and may contain
granules that stain with common blood neutrophils and cellular debris. An abscess
stains, such as Wright’s stain. These stains is an accumulation of purulent material
contain an acid dye, eosin, which is red, that has been isolated by the formation of
and a basic dye, methylene blue, which is surrounding connective tissue.
bluish. Granulocytes are named according Neutrophilia is an increase in the
to the color of the stained granules (i.e., number of circulating neutrophils in the
neutrophils, which have granules that blood that occurs with bacterial infec-
stain indifferently; eosinophils; and baso­ tions. Circulating neutrophils can increase
phils). The nuclei of granulocytes appear as a result of an immediate release of exist-
in many shapes and forms, leading to the ing neutrophils from the bone marrow and
name polymorphonuclear leukocytes an increase in neutrophil production.
(from the Greek poly, many; morpho, Neutropenia refers to an abnormally low
form). However, the term is commonly number of circulating neutrophils. The word
used to indicate neutrophils, because they endings ‐philia and ‐penia can be combined
are normally the most prevalent granulo- with the root names of the other leukocytes
cyte. Monocytes and lymphocytes are the to indicate similar conditions (e.g., eosino-
two types of agranulocytes.
philia is an increase in eosinophils).
Neutrophils. Neutrophils, a first line of
defense against infection, constitute a large Eosinophils. Eosinophils contain red‐
percentage of the total leukocyte number staining cytoplasmic granules. Normally,
(Table 15‐2). Upon tissue injury or eosinophils are less than 10% of total
microbial invasion, neutrophils rapidly leukocytes (Table 15‐2), but they may
accumulate within the interstitial fluids of increase in number with allergic conditions
the injured or invaded area. The active and parasitism. Eosinophils are ameboid
movement of neutrophils and other and somewhat phagocytic. Their primary
leukocytes out of intact blood vessels and function seems to be the regulation of
into the interstitial fluid is diapedesis. allergic responses and tissue responses to