Page 319 - Anatomy and Physiology of Farm Animals, 8th Edition

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fluid and plasma proteins by its actions on ability to have a fast, amplified response
after an initial exposure to an antigen.
blood vessels.
VetBooks.ir that viruses can replicate only within cells. These characteristics are extremely impor-
One unique aspect of viral infections is
tant, for they are the theoretical basis for
Viruses must enter cells of the animal’s vaccinations. Vaccination is essentially
body and use the cell’s own synthetic pro- induction of a specific immune response
cesses to form new viruses to infect other and immunologic memory by planned
cells. Two types of nonspecific defense exposure to an antigen in a manner that
mechanisms function to prevent viral does not produce disease.
infection and replication. Interferons are Lymphocytes are the essential leuko-
polypeptides produced and secreted by cytes that develop a specific immune
cells containing viruses, and almost all response. Their functions in this response
types of cells produce interferons after include: (1) antigen recognition; (2) anti-
being infected by viruses. Interferons are a body production; (3) cytotoxic attack on
means by which an infected cell can pre- infected cells; (4) immunologic memory;
vent further spread of the viral infection and (5) regulation of the specific immune
because they act on other cells in the area response. An individual lymphocyte does
to prevent viruses from using the synthetic not perform all of these functions, but
pathways of newly infected cells to pro- rather subpopulations or subtypes of lym-
duce new viruses. Viruses may enter cells phocytes are responsible for different
protected by interferons, but they cannot aspects of the specific immune response.
replicate within protected cells. Interferons In an attempt to generalize a very complex
are not specific for individual viruses, so process, this chapter describes the specific
interferons produced in response to immune response by considering the dif-
infection by one virus will protect against ferent types of lymphocytes and the roles
infection by a different virus. that each has in the overall response.
Natural killer (NK) cells are a specific
type of lymphocyte that can recognize and
destroy cells infected with viruses. This B Lymphocytes
recognition does not appear to be based on
viral antigens, so it is not specific for any B lymphocytes (B cells) are the lympho-
given virus. The NK cells destroy the cyte subtype associated with the produc-
infected cell by the secretion of substances tion of antibodies or the humoral
known as perforins and granzymes while component of a specific immune response
in direct contact with the infected cell. (Fig. 16‐1). The first step in the humoral
These secretions act on the cell membrane response is recognition of a foreign antigen
and enter the infected cell to destroy it. by B cells. This occurs when an antigen
While NK cells act in a nonspecific manner, (usually a protein as either a free molecule
their numbers are normally relatively low or a molecule on the surface of a cell mem-
and are only increased during a specific brane or a cell wall) binds to specific cell
immune response. membrane receptors on a selected subpop-
ulation of B cells. This subpopulation of B
cells (a clone) is the only group of B cells
Specific Immune Response with a membrane receptor capable of bind-
ing the antigen, and it is stimulated to pro-
Innate immune responses lack specificity liferate. The process by which a particular
and memory, two important characteris- subpopulation of B cells increases in num-
tics of specific immune responses. ber is termed clonal selection.
Immunologic specificity means that the For clonal section to be effective, each
response is directed at a specific antigen, animal must have a ready supply of lym-
while immunologic memory refers to the phocytes with unique membrane receptors