Immunology Continued
Specific Defenses of the Immune
System.
Specific Defenses of the Host
• Acquired Immunity: protection a person
develops over the course of one’s 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.
• 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.
– 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 individual’s body can
make its own antibodies.
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.
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.
• 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.
Antigen Binding Site
Variable Region
Light Chain
Constant Region
Hinge
Heavy Chain
General Structure of an antibody
• 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.
• The variable region is different within the
antibody class. So let’s 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.
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.
• 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 can’t occur until the cell with the right
antigen binding protein comes into contact with
the antigen. See Fig. 15.3.
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.
• 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.
• 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 You’ll 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, “don’t kill me, I belong here!”
• 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 can’t 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.
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.
• 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.
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”.
• Next Monday we will cover disorders of the
immune system and vaccination.
• No hw this time.