Page 310 - Anatomy and Physiology of Farm Animals, 8th Edition

Blood and Other Body Fluids / 295

parasites. They act by removing antigen– circulating leukocyte after neutrophils, but
they are more prevalent than neutrophils
antibody complexes, which stimulate
VetBooks.ir allergic responses, and by inhibiting some in ruminants (Table 15‐2). Based on their
functions, three general types or groups
of the mediators of allergic responses, such
as histamine. of lymphocytes have been identified
(B lymphocytes, T lymphocytes, and NK,
Basophils. Basophils contain blue‐ or natural killer cells), but these types
staining granules and are rarely seen in cannot be identified by differences in
normal blood. The granules of basophils appearance. In blood smears, lymphocytes
contain multiple compounds, including vary in size, with a relatively large nucleus
heparin, which prevents blood clotting, surrounded by a small amount of cytoplasm
and histamine, which relaxes smooth (see Fig. 1‐9). Lymphocytes function in
muscle of blood vessels and constricts specific immune responses and immune
smooth muscle in airways. Cells that are surveillance, and these functions are
very similar but distinct, mast cells, are discussed in Chapter 16.
found in many sites throughout the body
but are especially prevalent in connective Differential White Blood Cell
Counts. Differential counts indicate the
tissue below an epithelial lining exposed percentage of each type of white cell in the
to the external environment (e.g., dermis, blood sample (Table 15‐2). The various
wall of airways, and wall of gastrointestinal types of leukocytes have different functions
tract). Basophils and mast cells release the and respond differently to various types
contents of their granules during allergic of infections or diseases, so differential
responses, and these chemicals contribute counts can be useful for diagnosis.
to the characteristic tissue responses. The Differential counts taken over time can
release of the granules is mediated in part also be used to evaluate the response of an
by the binding of specific antibodies and animal to infection or disease. A differential
the associated allergen (antigen).
count is made by spreading a drop of whole
blood thinly on a glass slide to form a blood
Monocytes. Monocytes are the largest smear. The smear is dried and stained with
of the circulating leukocytes. They are a blood stain, such as Wright’s stain. After
phagocytic and develop into even larger staining is complete, the slide is examined
macrophages when they exit vessels and with a microscope and the number of
enter the tissues. Like neutrophils, white cells of each kind is tabulated until a
monocytes are attracted by chemotactic predetermined total number of white cells
factors to areas of tissue injury and microbial have been counted. The number counted
invasion. In addition to phagocytosis of is usually a multiple of 100, and the
tissue debris and microbes, macrophages percentage of each leukocyte type observed
have a major role in the overall initiation in a given sample of blood is called the
and regulation of inflammatory and differential leukocyte count or differential
immune responses. During their response white cell count. In reference laboratories,
to tissue injury or microbial invasion, both total red and white cell counts
macrophages release numerous chemical are semiautomatically determined by
messengers that coordinate the function of sophisticated laboratory equipment.
other cells responding to injury or invasion.
Macrophages also function in the processing
of antigens, a necessary step in the initiation
of an immune response. These functions Plasma and Serum
are discussed in more detail in Chapter 16.
When a sample of blood is treated with an
Lymphocytes. In most species, lympho­ anticoagulant to prevent clotting and
cytes are the second most prevalent permitted to stand in a tube undisturbed,