Immunology Continued

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Immunology Continued

• Specific Defenses of the Immune System.

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Specific Defenses of the Host

• Acquired Immunity protection a person develops
  over the course of ones life.
• 2 Types
• Passive antibodies are transferred from one
  person to another. In other words, one person
  acquired the disease or was vaccinated against
  the disease and the antibodies their body
  developed were transferred to another person. An
  example would be antibodies that an infant
  acquires through breast feeding. This kind of
  immunity lasts only as long as a continual source
  of the antibodies are present or as long as the
  antibodies themselves are present in the
  recipient.

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• Active An individual is exposed to
  microorganisms or foreign substances and the
  immune system responds by producing immune cells.
  This immunity is generally longer lasting and
  provides future protection against reinfection.
• These two types of immunity can be broken down
  further into the following categories
• Naturally acquired active immunity The
  individual is exposed to the microbe through
  contact in the environment and develops
  antibodies
• Naturally acquired passive immunity This is the
  natural transfer of antibodies from mother to
  child either through the placenta or through
  breast milk.

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• Artificially Acquired active immunity With this
  type of immunity, the individual is exposed to a
  weakened version of the microbe through a vaccine
  and develops antibodies.
• Artificially acquired passive immunity The
  individual acquires antibodies from an alternate
  source that is already immune to the disease, to
  fight infection until that individuals body can
  make its own antibodies.

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Antigens and Antibodies

• An antibody is a specialized protein that
  recognizes and interacts with a foreign substance
  called an antigen.
• Antibodies are made in B cells, specialized white
  blood cells.
• All antibodies are members of a group of proteins
  called immunoglobulins (Igs).
• An antigen is a protein or sugar chain that
  provokes a highly specific immune response
  because it is not recognized by the body.

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5 classes of antibodies

• IgG This antibody is capable of crossing the
  placenta to give the fetus immunity during
  gestation. The body produces a larger
  concentration of these antibodies upon the second
  exposure to an infectious agent.
• IgM The first antibodies made at the beginning
  of an infection.
• IgA This class of antibodies provides localized
  protection on mucousal membranes. This includes
  the respiratory tract, GI tract and is also the
  antibody passed through breast milk.

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• IgD The function of this antibody is not well
  understood at this time.
• IgE This antibody is responsible for histamine
  release during allergic reactions. Scientists
  are also finding that in countries where people
  are frequently exposed to intestinal parasites,
  that IgE seems to provide some protection against
  parasites. That function is not seen in most
  residents of the United States.

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Antigen Binding Site
Light Chain
Variable Region
Hinge
Constant Region
Heavy Chain
General Structure of an antibody
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• Antibodies are composed of light and heavy
  chains, or proteins. The light and heavy chains
  are bound together at the hinge.
• Both the light and the heavy chains are divided
  into a constant region and a variable region.
• The constant region means that that sequence of
  amino acids and protein structure remains the
  same for all antibodies within the same class.
  In other words, all IgG antibodies have the same
  constant region and all IgE antibodies have the
  same constant regions, but IgG antibodies do not
  have the same constant region as IgE antibodies.

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• The variable region is different within the
  antibody class. So lets say that a person has
  had both the chicken pox and the rubella vaccine.
  The IgG antibodies that their body made for each
  of those diseases has the same constant region,
  but the variable region is different. It is
  specific for the disease it protects against.
• The Antibody Binding Site is also specific for
  the microorganism that causes the disease. It
  recognizes and binds to the antigens made by the
  microorganism.

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2 Types of Immunity

• 1.Humoral Immunity antibody-mediated immunity.
• White Blood Cells called B cells are
  responsible for the production of antibodies.
• Antibodies primarily defends against bacteria,
  bacterial toxins, and viruses that circulate
  freely in body fluids.

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• 2. Cell-mediated immunity specialized white
  blood cells called T cells (T lymphocytes).
• Regulate activation and proliferation of other
  immune system cells such as macrophages.
• T cells are generally used to attack host cells
  containing viruses.
• Now before we continue, you need to understand
  that T cells and B cells come with specific
  anitgen binding proteins already on their
  membranes. Each cell is specific for a different
  antigen. This differentiation occurs before an
  individual is even born. That means that when a
  person is infected with a microorganism the
  activation of immune cells cant occur until the
  cell with the right antigen binding protein comes
  into contact with the antigen. See Fig. 15.3.

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T Cells and Cell-Mediated Immunity

• Mature T cells reside in lymphoid organs, such as
  the lymph nodes and spleen.
• Antigen Presenting Cells (APC), phagocytic cells,
  encounter an antigen in the body.
• The APC phagocytoses the foreign organism.
• Then it breaks it down, or processes the antigen.
  Essentially it disconnects part of the antigen
  from the rest of the cell and places it on the
  outside of its membrane. In this way it
  presents the antigen to other immune cells. It
  helps the other immune cells to see and recognize
  the antigen.

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• Helper T cell the binds to APC at the point where
  the antigen is presented on the membrane.
• The binding of the APC and the Helper T cell
  results in signal sent from the Helper T cell to
  Cytoxic T cells, macrophages, and B cells.
• See Figure 15.9 for a great diagram of this
  process.

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• Once the Cytotoxic T cells get the message from
  the Helper T cell, the Cytotoxic T cells destroy
  target cells on contact.
• Cytotoxic T cells destroy target cells by
  releasing perforin, an enzyme that forms pores
  (or punches holes) in the target cell membrane,
  causing it to lyse.
• Suppressor T cells are generally thought to turn
  off the immune response when it is no longer
  needed.
• In Fig. 15.16 Youll see two different kinds of
  APCs, MHC I and MHC II, that activate Helper T
  cells. MHC I and II are the proteins that mark
  that cell as belonging to the body. Essentially
  they mean, dont kill me, I belong here!

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• In addition to the activation of the cytotoxic T
  cells macrophages are also activated to help
  clean up the infection.
• B cells are activated to make antibodies by the T
  Helper cells. Often B cells cant begin antibody
  production without the help of the T Helper
  cells,
• Finally, memory T Cells are formed. They are the
  cells that remember a particular antigen when it
  is found in the body again and quickly begin
  making immune cells to prevent re-infection.

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B Cells and Humoral Mediated Immunity

• A specific B cell binds to the invading
  microorganism. It then processes it and presents
  the antigen in a way similar to the APC.
• For most B cells to become active they need help
  from the Helper T cells. The activated Helper T
  cell (previous activated through the APC) binds
  to the antigen presented on the B cell and
  activates the cell for antibody production.

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• Once the B cell has been activated it will make
  two different types of cells
• 1. Plasma cells These cells actively make
  antibody and release it to fight invading
  microorganisms.
• Antibodies help to fight infection in several
  ways.
• One way is by completely covering the invading
  microorganism with antibodies. Your book uses
  the analogy of providing a handle on a slippery
  object to help the phagocytes get a better grip
  on it.
• They can also work to bind many microorganisms
  together at one time until they can be
  phagocytosed.
• Antibodies also bind to the outside of viruses,
  covering all the attachment proteins on their
  outer surface preventing them from binding to
  host cells.

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• 2. Memory B cells These cells are made so that
  upon exposure to the same antigen in the future,
  antibodies can be quickly made before disease can
  occur.
• See Fig. 17.18 for a review of Ab mediated and
  cell mediated immune system and how they work
  together to protect the body.
• I find it amazing how much the body does to
  prevent infection on a daily basis. What is even
  more amazing is that microorganisms find ways to
  circumvent this amazing defense! I guess that
  leads us back to the constant struggle of
  survival of the fittest.

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• Next Monday we will cover disorders of the immune
  system and vaccination.
• No hw this time.