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UNIT IV
Infection, Inflammation, and Immunity
development of memory. Active immunity is usually long
lasting but requires a few days to weeks after a first expo-
sure to sufficiently develop an appropriate immunological
response that culminates in the destruction of the presenting
antigen. However, with subsequent exposure the immune sys-
tem rapidly becomes fully activated because of the presence
of memory B and T lymphocytes and circulating antibodies.
The process by which active immunity is acquired through
the administration of a vaccine is termed
immunization
. An
acquired immune response can improve on repeated exposures
to an injected antigen (booster vaccines) or a natural infection.
Passive immunity
is immunity transferred from another
source. The most common form of passive immunity is that
conferred from mother to fetus. During fetal development,
maternal IgG antibodies are transferred to the fetus via the
placenta. After birth, the neonate also receives IgG antibodies
from the mother in breast milk or colostrum. Therefore, infants
are provided with some degree of protection from infection for
approximately 3 to 6 months, giving their own immune sys-
tems time to mature. Some protection against infectious dis-
ease can also be provided by the administration of Igs pooled
from human or animal sources. Passive immunity produces
only short-term protection that lasts weeks to months.
Regulation of the Adaptive
Immune Response
In order for a host organism to remain healthy, the immune
system must function properly. A weakened immune response
may lead to immunodeficiency, but an inappropriate or exces-
sive response can cause allergic reactions and autoimmune
diseases. Therefore, the immune system must be capable of
regulating itself. The process by which the body regulates
itself is poorly understood but must involve all aspects of the
innate and adaptive immune responses.
Each exposure to an antigen elicits a predictable response
from the immune system. Once the immune system is acti-
vated, the response is amplified until it peaks and eventually
subsides. This occurs because the body’s normal immune
responses are self-limiting. Once the antigen is destroyed
and the action of chemical mediators terminated, the immune
response ceases. It is believed that anti-inflammatory cytokines
and regulatory T lymphocytes play a role in this process.
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Tolerance
also plays a role in the self-regulation of the
immune response. Tolerance is the ability of the immune sys-
tem to react to foreign antigens but remain nonreactive to self-
antigens. Tolerance to self-antigens protects the body from
harmful autoimmune responses. This is exquisitely impor-
tant in vital organs such as the brain, testes, ovaries, and eyes
where immunological damage could be lethal to the organism.
Many autoimmune diseases such as Hashimoto thyroid-
itis and insulin-dependent diabetes mellitus are caused by
impairment in both B and T lymphocyte (specifically cyto-
toxic lymphocytes) functions resulting in direct cellular dam-
age because the body immune system is no longer capable of
distinguishing “self” from “nonself.”
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Spleen
The spleen is a large, ovoid secondary lymphoid organ
located high in the left upper quadrant of the abdominal cav-
ity between the diaphragm and the stomach. The spleen filters
antigens from the blood and is important in the response to
systemic infections. It is divided into two systems: the white
pulp and the red pulp. The red pulp is well supplied with arter-
ies and venous sinusoids and is the area where senescent and
injured red blood cells are removed. The white pulp contains
lymphatic nodules and diffuse lymphoid tissue where con-
centrated areas of B and T lymphocytes permeated by mac-
rophages and DCs exist. The lymphocytes (primarily T cells)
that surround the central arterioles form the area called the
periarterial lymphoid sheath
. There is also a diffuse marginal
zone that contains the follicles and germinal centers and is
rich in B cells. This separates the white pulp from the red pulp
and allows lymphocytes to move easily between the blood and
the lymphatic tissue. A sequence of activation events similar
to that seen in the lymph nodes occurs in the spleen.
Other Secondary Lymphoid Tissues
Other secondary lymphoid tissues include the
mucosa-
associated lymphoid tissues
, which are nonencapsulated clus-
ters of lymphoid tissues located around membranes lining the
respiratory, digestive, and urogenital tracts. These organ sys-
tems constantly came in contact with pathogens and toxins
and, therefore, require the presence of immune cells in order
to respond to the potential invasion by pathogens and harmful
substances. In some tissues, the lymphocytes are organized
in loose, nondescript clusters, but in other tissues such as
the tonsils, Peyer patches in the intestine, and the appendix,
their structure is better organized. These tissues contain all
the cellular components (
i.e.,
T cells, B cells, macrophages,
and DCs) required to mount an immune response. Immunity
at the mucosal layers helps to exclude many pathogens from
the body and, as a result, protects the more vital internal
structures.
Active versus Passive Immunity
The goal of the immune system is to protect the host against
invasion by potentially dangerous pathogens, foreign sub-
stances, and other sources of harmful antigens. Adaptive
immune responses accomplish this goal through the activation
of cell-mediated and humoral responses. This type of protec-
tion can be induced in one of two ways:
1. After exposure to the offending substance and activa-
tion of B and T lymphocytes (active immunity)
2. Through the transfer of antibodies against an antigen
directly to the host (passive immunity)
Active immunity
is acquired when the host mounts an immune
response to an antigen either through the process of vacci-
nation or from environmental exposure. It is called
active
immunity
because it requires the host’s own immune sys-
tem to develop an immunological response including the
