Nonspecific Body Defenses
Specific Body Defenses: The Immune System
Developmental Aspects of the Lymphatic System
and Body Defenses
- The lymphatic system consists of the lymphatic vessels, lymph nodes,
and certain other lymphoid organs in the body (Figure 12.1).
- Extremely porous blind-ended lymphatic capillaries pick up excess
tissue fluid leaked from the blood capillaries (Figure
12.2). The fluid (lymph) flows into the larger lymphatics and finally
into the blood vascular system through the right lymphatic duct and
the left thoracic duct.
- Lymph transport is aided by the muscular and respiratory pumps
and by contraction of smooth muscle in the walls of the lymphatic vessels.
- Lymph nodes are clustered along lymphatic vessels, and the lymphatic
stream flows through them. Lymph nodes form agranular WBCs (lymphocytes),
and phagocytic cells within them remove bacteria, viruses, and the like
from the lymph stream before it is returned to the blood.
- Other lymphoid organs include the tonsils (in the throat), which
remove bacteria trying to enter the digestive or respiratory tracts:
the thymus, a programming region for some lymphocytes of the body; Peyer's
patches, which prevent bacteria in the intestine from penetrating deeper
into the body; and the spleen, a RBC graveyard and blood reservoir.
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Part 11: Body Defenses
- The first line of defense against pathogens are the surface membranes
(skin and mucous membranes). They provide mechanical barriers to pathogens.
Some produce secretions and/or have structural modifications that enhance
their defensive effects: The skin's acidity, lysozyme, mucus, keratin,
and ciliated cells are examples.
- Phagocytes (macrophages and neutrophils) engulf and destroy pathogens
that penetrate epithelial barriers. This process is enhanced when the
pathogen's surface is altered by attachment of antibodies and/or complement.
- Natural killer cells are nonimmune cells that act non-specifically
to lyse vims-infected and malignant cells.
- The inflammatory response prevents spread of harmful agents, disposes
of pathogens and dead tissue cells, and promotes healing (Figure
12.3). Protective leukocytes enter the area; the area is walled
off by fibrin and tissue repair occurs. The signs and symptoms of the
inflammatory response are: pain, redness, swelling, and heat.
- When complement (a group of plasma proteins) becomes fixed on the
membrane of a foreign cell, lysis of the target cell occurs. Complement
also enhances phagocytosis and tlie inflammatory and immune responses.
- Interferon is a group of proteins synthesized by virus-infected
cells and certain immune cells. It prevents viruses from multiplying
in other body cells.
- Fever enhances the fight against infectious microorganisms by increasing
metabolism, which speeds up repair processes; and by causing the liver
and spleen to store iron and zinc, which are needed for bacterial multiplication,
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- The immune system recognizes something as foreign and acts to inactivate
or remove it. Immune response is antigen-specific, is systemic, and
has memory. The two arms of immune response are humoral immunity, mediated
by antibodies, and cellular immunity. mediated by living cells (lymphocytes).
- Antigens are large, complex molecules (or parts of them) recognized
as foreign by the body. Foreign proteins are the strongest antigens.
- Complete antigens provoke an immune response and bind with
products of that response (antibodies or sensitized lymphocytes).
- Incomplete antigens, or haptens, are small molecules that are
unable to cause an immune response by themselves but do so when
they bind to body proteins and the complex is recognized as foreign.
- Cells of the immune system: An overview
- Two main cell populations, lymphocytes and macrophages, provide
- Lymphocytes arise from hemocytoblasts of bone marrow. T cells
develop immunocompetence in the thymus and oversee cell-mediated
immunity. B cells develop immunocompetence in bone marrow and provide
humoral immunity. Immunocompetent lymphocytes seed lymphoid organs,
where antigen challenge occurs, and circulate through blood, lymph,
and lymphoid organs.
- Immunocompetence is signaled by the appearance of antigen-specific
receptors on surfaces of lymphocytes.
- Macrophages arise from monocytes produced in bone marrow. They
phagocytize pathogens and present parts of the antigens on their
surfaces for recognition by T cells.
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- Humoral (antibody-mediated) immune response
- Clonal selection of B cells occurs when antigens bind to their
receptors, causing them to proliferate. Most clone members become
plasma cells, which secrete antibodies (Figure
12.4). This is called the primary immune response.
- Other clone members become memory B cells, capable of mounting
a rapid attack against the same antigen in subsequent meetings (secondary
immune responses). These memory cells provide immunological "memory."
- Active humoral immunity is acquired during an infection or
via vaccination and provides im-munoiogical memory. Passive immunity
is conferred when a donor's antibodies are injected into the bloodstream,
or when the mother's antibodies cross the placenta. It does not
provide immunological memory.
- Basic antibody structure. Antibodies are proteins produced
by sensitized B cells or plasma cells in response to an antigen,
and they are capable of binding with that antigen.
- An antibody is composed of four polypeptide chains (two heavy
and two light) that form a Y-shaped molecule (Figure
- Each polypeptide chain has a variable and a constant region.
Variable regions form antigen-binding sites, one on each arm of
the Y. Constant regions determine antibody function and class.
- Five classes of antibodies exist: IgA, IgG. IgM, IgD, IgE,
They differ structurally and functionally.
- Antibody functions include complement fixation, neutralization,
precipitation, and agglutination.
- Monoclonal antibodies are pure preparations of a single antibody
type useful in diagnosis of various infectious disorders and cancer,
and in treatment of certain cancers.
- Cellular (cell-mediated) immune response
- T cells are sensitized by binding simultaneously to an antigen
and a self-protein displayed on the surface of a macrophage. Clonal
selection occurs, and clone members differentiate into effector
T cells or memory T cells.
- There are several different classes of effector T cells. Cytotoxic
(killer) T cells directly attack and lyse infected and cancerous
cells. Helper T cells interact directly with B cells bound to antigens.
They also liberate lymphokines, chemicals that enhance the killing
activity of macrophages, attract other leukocytes, or act as helper
factors that stimulate activity of B cells and cytotoxic T cells.
Delayed hypersensitivity T cells release chemicals that enhance
inflammation and promote a delayed allergic reaction. Suppressor
T ceils terminate the normal immune response by releasing suppressor
chemicals (Figure 12.6).
- Disorders of immunity
- In allergy or hypersensitivity the immune system overreacts
to an otherwise harmless antigen, and tissue destruction occurs.
Immediate (acute) hypersensitivity, as seen in hayfever, hives,
and anaphylaxis, is due to IgE antibodies. Delayed hypersensitivity
(for example, contact dermatitis) reflects activity of T cells and
lymphokines, and nonspecific killing by activated macrophages.
- Immunodeficiencies result from abnormalities in any immune
element. Most serious is severe combined immunodeficiency disease
(a congenital disease) and AIDS, an acquired immunodeficiency disease
caused by a virus that attacks and cripples the helper T cells,
- Autoimmune disease occurs when the body's self-tolerance breaks
down, and antibodies and/or T cells attack the body's own tissues.
Most forms of autoimmune disease result from inefficient lymphocyte
programming in the fetus, changes in structure of self-antigens
or appearance of formerly hidden self-antigens in blood, and cross-reactions
with self-antigens and antibodies formed against foreign antigens.
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- Lymphatic vessels form by budding off veins. The thymus gland is
the first lymphoid organ to appear in the embryo. Other lymphoid organs
remain relatively undeveloped until after birth.
- Development of immune response occurs around the time of birth.
- The ability of immunocompetent cells to recognize foreign antigens
is genetically determined. Stress appears to interfere with normal immune
- Efficiency of immune response wanes in old age, and infections,
cancer, immunodeficiencies, and autoimmune diseases become more prevalent.