Biology 1202B Lecture Notes - Surface 3, Fetus, Immunity (Medical)

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8 Apr 2013
Lecture 23: Adaptive (Acquired) Immunity
Hematopoietic Stem Cells
Form all the blood cells
As well as the cells from the immune system
The key point here is that there are many different
kinds of cells that play a role within the immune
system and all of them originated from a stem cell.
The further away from the stem cell, the more
specialized these cells become.
Adaptive or Acquired Immunity
The third and most complex line of defense found only in vertebrates
Reaction is specific to foreign substance
Foreign molecules can be free, fund on the surface of a virus, cancer cells, pollen
or transplanted organs
Attack is to neutralize or eliminate pathogen
Reaction is triggered by specific molecules on the pathogen that are recognized
as being foreign “nonself” to the body, since it is specific it may take several days
to become effective.
Body retains a memory of the first exposure to that pathogen, enabling it to
respond more quickly if the pathogen is encountered again in the future
An antigen is any foreign substance (exogenous or endogenous) that can elicit
an adaptive immune response
o Endogenous generated within the body, such as cancer cells
o Exogenous enter from outside of the body, such as a virus, bacteria or
Antigen specifically means “antibody generator”
Most antigens are macromolecules:
o Large proteins (glycoproteins and lipoproteins)
o or polysaccharides (lipopolysaccharides)
o Can be nucleic acid or synthesized molecules
Antigens are recognized in the body by two different types of lymphocytes B
lymphocytes (B cells) and T lymphocytes (T cells)
B cells
Differentiate from stem cells in the bone marrow
After they differentiate, thy are released into the blood and carried to capillary
bed serving the tissues and organs of the lymphatic system
T cells
Produced by the division of stem cells in the bone marrow
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Released into the blood and carried to the thymus (an organ of the lymphatic
system) where they differentiate
How do we recognize that lymphocytes recognize antigens?
Experiments showed that leukocytes (WBC) in mice were killed by irradiation.
These mice couldn’t develop an adaptive immunity
Injecting lymphocytes from normal mice into these irradiated mice restored the
adaptive immune response
Other bodily cells did not restore the response
o Ultimately, this experiment shows that lymphocytes recognize antigens
because when no lymphocytes are present the mice do not have
Two Types of Adaptive Immune Responses
1. Antibody mediated immunity (AMI) or humoral immunity
2. Cell-mediated immunity
AMI and CMI have similar mechanisms
Lymphocyte recognizes and binds to antigen
Lymphocyte divide to produce a large number of clones
Activated lymphocyte clears antigen from body
Activated lymphocytes differentiate into memory cells that circulate in the blood
and lymph ready to initiate a rapid immune response upon seeing the same
Antibody Mediated Immunity
Antibody is a protein produced by the body in adaptive immunity to destroy or
neutralize an antigen
Antibody Recognition
Each B and T cell has thousands of antigen specific, identical receptors on its
plasma membrane. Called B-cell receptors (BCR’s) and T-cell receptors (TCR’s)
Each B and T cell (with multiple identical receptors) can bind to only one
specific antigen, but the entire population of B & T cells in a body can collectively
recognize millions of antigens
o Each antigen can be recognized by many B & T cells
o For example, we each have about 10 trillion B cells that collectively have
about 100 million different kinds of BCRs. These BCRs are formed even
before the body has encountered an antigen.
Antibody Binding
BCR or TCR do not bind to an entire antigen
Instead, the variable region of BCR and TCR bind to a portion of the antigen
called the antigenic determinant or epitope
o You can think of this portion as just the fingers or toes on a person
o The variable will bind to just the fingers, not the whole body.
Therefore, several different B and T cells may bind to different portions of a
particular antigen
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