CHAPTER OUTLINE
13.1 The Lymphatic System
The lymphatic system consists of lymphatic vessels and the lymphoid organs. It is closely
associated with the cardiovascular system.
Lymphatic Vessels
Lymphatic vessels form a one-way system that begins with lymphatic capillaries, tiny,
closed-ended vessels whose walls consist of simple squamous epithelium. Lymphatic
capillaries absorb excess tissue fluid. The fluid inside lymphatic vessels is called lymph.
Lymphoid Organs
Lymphoid organs contain large numbers of lymphocytes, the type of white blood cell
that is mainly responsible for adaptive immunity. There are two types of lymphoid
organs.
Primary Lymphoid Organs
The red bone marrow and thymus are primary lymphoid organs. Red bone
marrow is the site of stem cells that are ever capable of dividing and producing
blood cells. Immature T cells migrate from the bone marrow through the blood
stream to the thymus, where they mature. Only about 5% of T cells ever exit the
thymus and those that do are capable of reacting to foreign molecules.
Secondary Lymphoid Organs
Some lymphocytes become activated in secondary lymphoid organs like the
lymph nodes and spleen. Lymphocytes migrate to the secondary lymphoid
organs. Here they encounter foreign molecules or cells, after which they
proliferate and become activated. The spleen filters the blood. Lymph nodes are
packed with B and T cells that help the body fight infection.
13.2 Innate Immunity
The lymphatic systems works with the immune system to protect the body from pathogens,
toxins, and other invaders. The term immunity refers to a condition where the body is protected
from various threats, like pathogens, toxins and cancer cells. Mechanisms of innate immunity
are fully functional without previous exposure to these substances, whereas adaptive immunity is
initiated and amplified after specific recognition of these substances. Mechanisms of innate
immunity can be divided into at least four types: physical and chemical barriers, inflammation,
phagocytes and natural killer cells, and protective proteins.
Physical and Chemical Barriers
Skin and mucous membranes lining the respiratory, digestive, and urinary tracts
serve as mechanical barriers to entry of pathogens. The secretions of oil glands
in the skin contain chemicals that weaken or kill certain bacteria on the skin. The
stomach has an acid pH, which kills many types of bacteria or inhibits their
growth.
Inflammatory Response
Damage to tissues, whether by physical trauma, chemical agents, or pathogens, initiates a
series of events known as the inflammatory response. An inflamed area has four signs:
redness, heat, swelling, and pain.
Phagocytes and Natural Killer Cells
At sites of inflammation, various types of white blood cells migrate through the walls of
dilated capillaries, into the inflamed area. Two types of phagocytes, neutrophils and
macrophages engulf pathogens. Natural killer cells kill some virus-infected and cancer
cells by cell-to-cell contact.
Protective Proteins
The complement system is composed of a number of blood plasma proteins that
“complement” certain immune responses. Interferons are proteins produced by virusinfected cells as a warning to noninfected cells.
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13.3 Adaptive Immunity
When innate defenses fail to prevent an infection, adaptive immunity comes into play.
The adaptive immune system recognizes, responds to, and usually eliminates antigens
from the body. An antigen is any molecule that stimulates an adaptive immune response.
Antigen Receptor Diversity and Clonal Selection
The adaptive immune system depends primarily on the activity of B cells and T cells,
which are capable of recognizing antigens because they have specific antigen receptors.
The immune system is able to distinguish “self” from “nonself.”
B Cells and Antibody-Mediated Immunity
During clonal expansion B cells divide and most of the resulting clones become
plasma cells, specialized for the secretion of antibodies. Some cloned B cells
become memory B cells, which are the means by which long-term immunity is
possible. Defense by B cells is called antibody-mediated immunity.
Antibody Structure
Antibodies are also called immunoglobulins. They are typically Yshaped molecules with two antigen-binding sites.
Types of Antibodies
There are five major classes of antibodies in humans: IgG, IgM, IgA,
IgD, and IgE (Table 13.1).
T Cells and Cell-Mediated Immunity
T cells can only recognize an antigen when it is displayed to them by an MHC
(major histocompatibility complex) protein on the surface of another cell.
There are two major types of T cells: helper T cells and cytotoxic T cells.
Helper T cells secrete various cytokines that activate cytotoxic T cells, which
often kill the cells they recognize. Immunity mediated by T cells is sometimes
referred to as cell-mediated immunity.
13.4 Active vs. Passive Immunity
In active immunity, the individual alone produces an immune response against an antigen. In
passive immunity, the individual receives antibodies or cells from another individual.
Active Immunity
Active immunity usually develops naturally after a person is infected with a pathogen.
Immunization involves the use of vaccines, preparations that contain an antigen to
which the immune system responds, to initiate immunity.
Passive Immunity
Passive immunity occurs when an individual receives another individual’s antibodies or
immune cells. The passive transfer of antibodies is a common natural process between
newborns and their mothers.
Immune Therapies
Cytokines and Immunity
Cytokines are chemical messengers produced by T cells, macrophages, and other
cells. They regulate white blood cell formation and/or function and are used as
cytokine therapy in the stimulation of blood cell production and the treatment of
cancer. One serious drawback with cytokine therapy is the risk of side effects,
which can be life-threatening.
Monoclonal Antibodies
Monoclonal antibodies are antibodies of the same type that are produced by
plasma cells derived from the same B cell, they can be produced in vitro.
Monoclonal antibodies can be used for quick and certain diagnosis of various
conditions. These may be used to treat cancer.
13.5 Adverse Effects of Immune Responses
Sometimes the immune system responds to harmless antigens in a manner that damages the body.
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Allergies
Allergies are hypersensitivities to substances that ordinarily would do no harm to the
body. The response to these antigens, called allergens, usually includes some degree of
tissue damage. An immediate allergic response can occur within seconds of contact
with the antigen and may result in anaphylactic shock, a life-threatening problem. A
delayed allergic response is initiated by memory T cells.
Blood-Type Reactions
The ABO system is the most important blood typing system.
ABO System
The presence or absence of type A and type B antigens on red blood cells
determine a person’s blood type. The ABO system identifies four types of blood:
A, B, AB, and O. Transfusion with the wrong type blood can be fatal.
Rh System
Another important antigen in matching blood types is the Rh factor. A mismatch
can be fatal.
Tissue Rejection
The recipient’s immune system attempts to reject transplanted organs. MHC proteins can
serve as antigens when organs of a different MHC type are transplanted into a recipient.
The chance of organ rejection can be diminished by administering immunosuppressive
drugs. Xenotrasplantation is the use of animal organs instead of human organs in
transplant patients.
13.6 Disorders of the Immune System
When a person has an autoimmune disease, cytotoxic T cells or antibodies mistakenly recognize
the body’s own cells or molecules as if they are foreign antigens. Examples include myasthenia
gravis, multiple sclerosis, systemic lupus erythematosus and rheumatoid arthritis. When a
person has an immunodeficiency disease, the immune system is unable to protect the body
against disease.
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