The Immune System

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The Immune System

• Philip L. Pokorski, Ph.D.
• Department of Biology
• University of Michigan--Dearborn

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Nonspecific Defenses

• The immune system includes cells and tissues that
  are responsible for immunity.
• Immunity is the bodys ability to defend against
  infection and involves nonspecific and specific
  defenses.
• The nonspecific defenses are effective against
  many types of infectious agents.

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Barriers to Entry

• Skin and mucous membranes lining the respiratory,
  digestive and urinary tracts, oil glands in the
  skin, ciliated cells that sweep particles in
  mucus, and an acidic stomach all contribute to
  keeping pathogens from entering the body.
• Beneficial bacteria in the intestines and vagina
  also prevent pathogens from taking up residence.

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Inflammatory Reaction

• The inflammatory reaction involves redness, heat,
  swelling, and pain.
• The release of histamine and kinins from damaged
  tissue cells and from mast cells causes redness
  and swelling.
• The swollen area and kinins stimulate free nerve
  endings, causing the sensation of pain.

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• Macrophages migrate to the site of injury and can
  engulf pathogens and also release
  colony-stimulating factors that cause the bone
  marrow to release more white blood cells.
• Anti-inflammatory drugs combat chronic
  inflammation by acting against chemical mediators
  released by white blood cells.
• The presence of pus indicates the body is trying
  to overcome the infection.

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Inflammatory reaction
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Natural Killer Cells

• Natural killer cells kill virus-infected cells
  and tumor cells by cell-to-cell contact.
• They are large, granular lymphocytes with no
  specificity and no memory.
• Their number is not increased by prior exposure
  to that kind of cell.

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Specific Defenses

• An antigen is any foreign substance that
  stimulates the immune system to react to it.
• The body does not consider its own proteins
  foreign therefore the immune system must
  distinguish self from nonself.
• Lymphocytes have a large number of antigen
  receptors.

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• Specific defenses require B lymphocytes (B cells)
  and T lymphocytes (T cells), which are both
  produced in the bone marrow however, T cells
  mature in the thymus, while B cells mature in
  bone marrow.
• B cells give rise to antibodies that are shaped
  like antigen receptors and are capable of
  combining with and neutralizing antigens.
• T cells do not produce antibodies but instead
  attack foreign antigens directly.

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B Cells and Antibody-Mediated Immunity

• A toxin is a chemical produced by certain
  bacteria that is poisonous.
• As a B cell encounters a bacterial cell or a
  toxin with a specific antigen in a lymph node or
  spleen, it is activated to divide.
• The resulting cells are plasma cells, mature B
  cells that mass-produce antibodies.
• Defense by B cells is thus called
  antibody-mediated immunity.

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Structure of IgG

• The most common type of antibody, the IgG
  antibody, is a Y-shaped molecule that has two
  binding sites for a specific antigen.
• Antigen-antibody complexes often mark the antigen
  for destruction by neutrophils or macrophages, or
  they may activate complement.

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Structure of IgG
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Other Types of Antibodies

• There are five types of antibodies
• IgG the main type in circulation, binds to
  pathogens, activates complement, and enhances
  phagocytosis
• IgM the largest type in circulation, activates
  complement and clumps cells
• IgA found in saliva and milk, prevents
  pathogens from attaching to epithelial cells in
  digestive and respiratory tracts

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• IgD on surface of immature B cells, its
  presence signifies the readiness of a B cell
• IgE found as antigen receptor on basophils in
  blood and on mast cells in tissues, responsible
  for immediate allergic response and protection
  against certain parasitic worms.
• The different classes of antibodies vary in
  structure.

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Types of T Cells

• Cytotoxic T cells kill infected cells that bear a
  foreign antigen on contact. Cytotoxic T cells
  provide cell-mediated immunity.
• Helper T cells stimulate other immune cells and
  produce cytokines.
• Some T cells are memory T cells that will
  jump-start an immune reaction upon re-infection.

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Induced Immunity

• Immunity occurs naturally by infection or is
  induced by medical intervention.
• The two types of induced immunity are active
  immunity and passive immunity.
• In active immunity, the individual produces the
  antibodies against an antigen.
• In passive immunity, the individual is given
  prepared antibodies.

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Active Immunity

• A person naturally develops active immunity after
  infection.
• Immunization involves the use of vaccines,
  substances that contain an antigen to which the
  immune system responds.
• Vaccines are available to induce long-lived
  active immunity in a well person.

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• After exposure to a vaccine, which is a
  non-virulent disease agent, antibodies are
  produced.
• With a booster shot or second exposure, the
  antibody titer rises to a much higher level.
• Active immunity is long-lived because there are
  memory B cells and memory T cells that will
  respond to lower doses of antigen in the body.

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Fig. 14.9ab
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Passive Immunity

• Passive immunity occurs when an individual is
  given prepared antibodies.
• For example, a newborn has antibodies that passed
  from its mother through the placenta.
• Breast-feeding passes antibodies from mother to
  child.
• However, passive immunity is short-lived since
  the antibodies were not produced by the persons
  own B cells.

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Cytokines and Immunity

• Cytokines are signaling molecules produced by T
  lymphocytes, monocytes, and other cells.
• Both interferon and interleukins are cytokines
  used to improve a persons own T cell performance
  in fighting cancer.
• Interleukins show promise in the treatment of
  chronic infectious diseases.

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Monoclonal Antibodies

• All plasma cells derived from the same B cell
  secrete an identical antibody.
• B lymphocytes can therefore be exposed to a
  particular antigen and will produce monoclonal
  antibodies to the specific antigen.

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Immunity Side Effects

• The immune system usually protects us from
  disease because it can distinguish self from
  nonself.
• Sometimes, however, it responds in a manner that
  harms the body.
• Examples include allergies, tissue-rejection
  reactions, or autoimmune diseases.

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Allergies

• Allergies are hypersensitivities to substances
  such as pollen or animal dander that are normally
  not harmful.
• Weak antigens such as these are called allergens.
• The response itself can cause some degree of
  tissue damage.

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Immediate Allergic Response

• An immediate allergic response can occur within
  seconds after contacting an antigen.
• Anaphylactic shock is a severe reaction
  characterized by a sudden life-threatening drop
  in blood pressure.
• Immediate allergic responses are caused by IgE
  antibodies attaching to mast cells and basophils
  these cells then release histamine which causes
  the symptoms of allergies, some of which are
  severe.

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Delayed Allergic Response

• Delayed allergic responses are started by memory
  T cells at the site of the allergen
• .
• The response is regulated by cytokines secreted
  by T cells and macrophages
• .
• Examples of delayed allergic responses include
  the skin test for tuberculosis and contact
  dermatitis from poison ivy, jewelry, and other
  possible irritants.

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Blood-Type Reactions

• Illness and death sometimes resulted from the
  first attempts at blood transfusions.
• It was later discovered that only certain types
  of blood are compatible because red blood cell
  membranes carry proteins or sugar residues that
  are antigenic to recipients.
• The ABO blood system represents a series of
  antigens on red blood cells that denote blood
  type.

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ABO System

• The ABO blood typing system is based on the
  presence of two antigens on the surface of red
  blood cells antigen A and antigen B.
• Blood types include A, B, or AB, or type O, which
  has no antigens.
• In the plasma there are two possible
  naturally-occurring antibodies anti-A and
  anti-B.

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• If the corresponding antigen and antibody are put
  together, clumping, or agglutination, occurs in
  this way the blood type of the individual may be
  determined.

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Blood typing
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Rh System

• The Rh factor is also a blood type a person
  with this antigen on their red blood cells is Rh
  positive (Rh) those without it are Rh negative
  (Rh-).
• Rh factor is important during pregnancy because
  an Rh- mother may form antibodies to the Rh
  antigen during the pregnancy or at the birth of a
  child who is Rh.

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• These antibodies can cross the placenta to
  destroy the red blood cells of any subsequent Rh
  child, causing hemolytic disease of the newborn.
• A Rho-Gam injection uses anti-Rh antibodies to
  attack Rh cells before they can stimulate the
  mother to produce her own antibodies.

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Hemolytic disease of the newborn
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Tissue Rejection

• Tissue rejection occurs when cytotoxic T cells
  bring about the destruction of foreign tissue in
  the body.
• A close match between donor and recipient can
  reduce rejection.
• Immunosuppressive drugs act by inhibiting the
  response of T cells to cytokines, but can result
  in kidney damage.

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Autoimmune Diseases

• Autoimmune disease occurs when cytotoxic T cells
  or antibodies mistakenly attack the bodys own
  cells as if they bear foreign antigens.
• Autoimmune diseases include myasthenia gravis,
  multiple sclerosis, systemic lupus erythematosus,
  and rheumatoid arthritis.
• It has been suggested that type I diabetes and
  heart damage after rheumatic fever are autoimmune
  diseases.