Transcript The Immune System - Mrs.C's Web Page

The Immune System
By Max Soghikian and Frank Yü
Immune System: Overview
• Animals must not only
protect themselves from
predators but also
pathogens.
– An animal’s collective
defense against pathogens
is known as the immune
system.
• All animals have an
innate immunity
– Vertebrates also have
acquired immunity
Phagocyte engulfing a bacterium
Innate Immunity
• Innate Immunity
is found in all
animals
– It is active either
constantly, or
immediately
when a
pathogen is
discovered.
• Innate Immunity
is divided into:
– barrier defenses
(i.e. the skin)
– internal
defenses (i.e.
phagocytic cells)
The human skin (left) and a typical phagocyte (right)
Barrier Defenses
• Barrier Defenses are the first
line of an animal’s innate
immunity. They include:
– The skin, which physically
blocks most pathogens from
entering the body
– Mucous membranes, which
secrete a viscous fluid (known
as mucus) that traps microbes
and other harmful particles.
– Other body secretions, such
as tears, oil, and sweat also
prevent the growth of many
harmful microorganisms.
In the trachea, ciliated cells sweep mucus and
the stuff trapped in them away from the lungs.
Internal (Cellular) Defenses
•
Internal Defenses tend to activate
as soon as a pathogen enters the
body.
– Many times, the defenses are
triggered by TLRs (toll-like
receptors) that recognize various
characteristics of microbes.
•
These can be cell based.
– Neutrophils, the most abundant
phagocytic cells, are attracted to
infected tissues.
– Macrophages patrol the
bloodstream for foreign particles
and organisms and reside in
lymph nodes
– Eosinophils are specialized cells
that attach to multicellular
invaders and secrete destructive
enzymes
– Dendritic cells stimulate acquired
immunity against encountered
microbes
Phagocytosis
Internal (Protein) Defenses
• After a pathogen is recognized by the
organism, various antimicrobial proteins
and peptides attack foreign organisms in
various ways.
– Interferons are proteins that disrupt viral
infection and help contain exposure in the
body
– The complement system consists of some
30 or so proteins that, when activated, cause
invading cells to undergo lysis (burst).
Internal (Inflammatory) Defenses
• When the body recognizes that it’s being attacked, the area in
question undergoes various changes.
– For example, when near microinvaders, mast cells release histamine, a
hormone, which, along with the secretions from endothelial cells,
activates various cell reception cascades that make the blood vessels
more permeable and attract various phagocytic cells.
– An example of an inflammatory response that affects the entire body is
fever, which may encourage phagocytosis and tissue repair.
Pathogen
Mast cell
Splinter
Chemical Macrophage
Signals
(i.e. Histamine)
Capillary
Red blood cells Phagocytic cell
Fluid
Phagocytosis
Internal (NK) Defenses
• Natural Killer Cells are cells that are
alerted to infected cells by the Class I
MHC molecule.
– The NK cells then proceed to cause
infected/cancerous cells to undergo apoptosis
to limit damage
Acquired Immunity
•
Acquired Immunity is a branch of immunity found only in vertebrates.
–
–
–
It is centered around lymphocytes which are split into T Cells and B Cells, which act as
memory cells that record and store information about various pathogens
This is known as immunological memory, which results in an enhanced defense against a
repeat attacker
Foreign organisms are recognized by antigens, which is any molecule that is recognized
and remembered by a lymphocyte
Antigenbinding
site
Antigenbinding site
Antigenbinding
site
Disulfide
bridge
C
C
Light
chain
Variable
regions
V
V
Constant
regions
C
C
Transmembrane
region
Plasma
membrane
Heavy chains
 chain
 chain
Disulfide bridge
B cell
(a) B cell receptor
Cytoplasm of B cell
Cytoplasm of T cell
(b) T cell receptor
T cell
Acquired Immunity (Antigen Receptors)
• The cell membrane of a lymphocyte is coated with these
receptors.
– The parts of the antigen that binds to the binding sites are known
as epitopes
Antigenbinding
sites
Antigen-binding sites
Antibody A Antigen Antibody C
C
C
Antibody B
Epitopes
(antigenic
determinants)
Acquired Immunity (B Cells)
• B Cells have two antigen
binding sites formed by
heavy and light chains.
– They also can form plasma
cells, which secrete
antibodies (also known as
immunoglobin proteins)
which are essentially
mobile/soluble antigen
receptors
– The antigen receptors of B
cells form the well known
Y-shape of antibodies
Acquired Immunity (T Cells)
•
•
The antigen receptors of T Cells
have two heavy chains, labeled
alpha and beta.
T Cells tend to work in conjunction
with major histocompatibility
complexes (MHCs)
– When cells are infected by a
particular pathogen, they present the
corresponding antigen with either a
Class I or Class II MHC molecule
– The antigen receptors of T cells thus
bind to these presented antigen and
execute their various tasks
• For example, cytotoxic T Cells
destroy infected cells displaying the
MHC molecule with antigen attached
– Class II MHCs are used by more
specialized cells and often are
binded to by helper T Cells, which
assist B and cytotoxic T Cells
Acquired Immunity (Primary response)
• One of the most
important abilities of
acquired immunity is the
ability to fight repeat
attackers with greater
strength
– In primary immune
response, which peaks
about 10 to 17 days after
initial exposure, plasma
cells, other effector cells
and memory cells are
cloned with “memories” of
the particular antigen.
Acquired Immunity (Secondary Response)
• After an immune system has
gone through primary
response, the next exposure to
the antigen will result in a
much stronger response
because cloned memory cells
will be able to further assist
phagocytic and other types of
immunity
• Because memory cells are
relatively long lived, secondary
response can be activated
years after primary response in
some cases
Acquired Immunity (Humoral Response)
• Humoral immune response
involves the cloning of B
effector cells, known as plasma
cells, secrete large amounts of
antibodies which cover
pathogens, disabling some of
their harmful functions and
making them easier targets for
phagocytosis
• After primary response, the B
cell clones itself into memory B
cells and the body is prepared
against a repeat attack
Acquired Immunity (Cell-Mediated Response)
• Cell-mediated response
centers around cytotoxic T
cells, which seek and destroy
infected cells that display
antigens with MHC molecules
• As stated before, cytotoxic T
cells cause apoptosis in
infected cells.
• Eventually, surviving T cells
clone into memory T Cells,
much like memory B cells in
that they preserve
Immunological Memory for
years to come
Acquired Immunity (Overview of Humoral
and Cell-Mediated Reponse)
Antigenpresenting
cell
Peptide antigen
Bacterium
Class II MHC molecule
CD4
TCR (T cell receptor)
Helper T cell
Humoral
immunity
(secretion of
antibodies by
plasma cells)
Cytokines
+
B cell
+
+
+
Cytotoxic T cell
Cell-mediated
immunity
(attack on
infected cells)
Acquired Immunity (Antibody classes)
• There are various
types of antibodies
used by the immune
system
• Their varying
structures all allude to
their respective
functions.
Acquired Immunity (Active Immunity)
• Active Immunity is
essentially the result
of primary response.
– Cloned memory cells
of both the T and B
line will allow for
strong secondary
responses long into
the future.
– Vaccination is used
as a way to confer
active immunity
Acquired Immunity (Passive Immunity)
• Because the immune
system of most
mammalian infants are
not quite up to their full
protective potential,
passive immunity can be
extended to the child
through the mother
– The two most common
ways this occurs is through
breast feeding and through
the placenta of a pregnant
mother
– These two Ig classes assist
the child in fending off
possible infection
Various Diseases and Complications
• The immune system is the body’s defense against
pathogenic disease. Thus, when the immune system is
misfiring or weakened, serious/annoying problems can
occur.
– Allergies are when the immune system overreacts to a harmless
particle
• This can result in irritation and in serious cases, death
– Autoimmune diseases occur when the immune system fails to
determine between self and nonself, instead, attacking all cells.
– Immunodeficiency syndrome is when the immune system itself is
weakened by a pathogen.
• The most well known pathogen that causes this is HIV (human
immunodeficiency virus).
– A mis-functioning immune system can also lead to cancer, if
cytotoxic T cells cannot destroy cancerous cells well enough
References
• AP Biology Textbook
• http://en.wikipedia.org/wiki/Antibody_opso
nization
• http://en.wikipedia.org/wiki/Skin
• http://en.wikipedia.org/wiki/Basophil_granu
locyte
• http://en.wikipedia.org/wiki/Antibody