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UNIT IV
Infection, Inflammation, and Immunity
The adaptive immune system is the final line of defense against
infection and is activated once the innate immune response
initiates the inflammatory process. In contrast to innate immu-
nity, the adaptive immune response is capable of targeting
specific cells or organisms that it recognizes as foreign to
the body through activation of various lymphocytes and their
products, including antibodies. The lymphocytes involved in
adaptive immunity have the unique ability to remember spe-
cific pathogen and mount a heightened immune response dur-
ing repeat exposures. Each exposure results in a more rapid
and aggressive response. Substances present on the surface
of pathogens or other foreign substances that elicit adaptive
immune responses are called
antigens
. Adaptive immunity
involves two distinct but interconnected mechanisms: humoral
and cell-mediated responses.
Humoral immunity
is mediated
by
B-lymphocyte
activation and subsequent antibody produc-
tion. It is the primary defense against extracellular microbes
and toxins. In contrast,
cell-mediated immunity
involves the
activation of specific
T lymphocytes
(T-helper and T-cytotoxic
lymphocytes), which are responsible for the body’s defense
against intracellular microbes such as viruses.
Key Points
ADAPTIVE IMMUNITY
•
The adaptive immune response involves a com-
plex series of interactions between components of
the immune system and the antigens of a foreign
pathogen. It is able to distinguish between self and
nonself, recognize and specifically react to large
numbers of different microbes and pathogens, and
remember the specific agents.
•
Humoral immunity consists of protection provided
by the B lymphocyte–derived plasma cells, which
produce antibodies that travel in the blood and
interact with circulating and cell surface antigens,
whereas cell-mediated immunity provides pro-
tection through cytotoxic T lymphocytes, which
protect against virus-infected or cancer cells.
Antigens
Antigens,
or
immunogens
, are substances or molecules that are
foreign to the body but when introduced trigger the produc-
tion of antibodies by B lymphocytes leading to the ultimate
destruction of the invader. They are usually large macromol-
ecules (>10,000 Da) such as proteins, polysaccharides, lipids,
and free nucleic acids. Antigens are recognized by specific
receptors present on the surface of lymphocytes and by the
antibodies
or
immunoglobulins
secreted in response to the
antigen. Antigens can take the form of any foreign substance
including bacteria, fungi, viruses, protozoa, parasites, and
nonmicrobial agents such as plant pollens, insect venom, and
transplanted organs.
IN SUMMARY
The innate immune system is a complex system that works
in an organized, rapid, yet nonspecific fashion as the
body’s first line of defense against invasion. It is comprised
of the epithelial cells of the skin and mucus membranes;
phagocytic cells such as the neutrophils, macrophages,
and NK cells; and a series of plasma proteins including
cytokines, chemokines, and the proteins of the complement
system. These defenses exist before the body encounters
an invading microorganism and are activated independent
of the adaptive humoral response. The epithelial cells of
the skin and mucous membranes block the entry of infec-
tious agents and secrete antimicrobial enzymes, proteins,
and peptides in an attempt to prevent microorganisms from
invading the internal environment.
The phagocytes of the innate immune response engulf
and digest infectious agents. They utilize PRRs, which are
present on their membranes to recognize and bind broad
patterns of molecules (PAMPs) shared by microbes and
that are essential for their survival. TLRs are the most stud-
ied of all PRRs and are expressed on many of the cells of
the innate immune system. TLRs are involved in responses
to widely divergent types of molecules that are commonly
expressed by microbial but not mammalian cell types.
Development of a healthy innate immune response is
dependent not only upon the coordinated activity of the leuko-
cytes but on the secretion of chemical mediators and soluble
molecules, such as opsonins, cytokines, acute-phase proteins,
and complement. Opsonins bind to and tag microorganisms
for more efficient recognition by phagocytes. Activated leu-
kocytes release cytokines that stimulate the migration of leu-
kocytes to the site of inflammation, stimulate production of
acute-phase proteins, and enhance phagocytosis.
The complement system is a primary effector system that
functions as part of both the innate and adaptive immune
responses. It is comprised of a group of proteins that are
activated by three distinct but convergent pathways: the
classical, the lecithin, and the alternative pathways. The
primary function of the complement system is the promo-
tion of inflammation and destruction of the microbes.
ADAPTIVE IMMUNITY
After completing this section of the chapter, you should
be able to meet the following objectives:
••
Characterize the significance and function of major
histocompatibility complex molecules.
••
Compare and contrast the development and function
of the T and B lymphocytes.
••
Describe the function of cytokines involved in the
adaptive immune response.
