Transcript The Immune System

The Immune
System
Leucocytes
Lymphatic system
INNATE IMMUNITY
• The innate immunity system is what we
are born with and it is nonspecific; all
antigens are attacked pretty much
equally. It is genetically based and we
pass it on to our offspring.
Surface Barriers or Mucosal
Immunity
1.The first and most important
barrier is the skin. The skin
cannot be penetrated by most
organisms unless it already has
an opening, such as a nick,
scratch, or cut.
2. Pathogens are expelled from the lungs by
ciliary action as the tiny hairs move in an
upward motion; coughing and sneezing
abruptly eject both living and nonliving things
from the respiratory system; the flushing
action of tears, saliva, and urine also force
out pathogens, as does the sloughing off of
skin.
3. Sticky mucus in respiratory and
gastrointestinal tracts traps many
microorganisms.
4. Acid pH (< 7.0) of skin secretions inhibits
bacterial growth. Hair follicles secrete sebum
(contains lactic acid and fatty acids) that
inhibit the growth of some pathogenic
bacteria and fungi. Areas of the skin not
covered with hair, such as the palms and
soles of the feet, are most susceptible to
fungal infections. (Think athlete's foot.)
5. Saliva, tears, nasal secretions, and
perspiration contain lysozyme, an enzyme
that destroys bacterial cell walls causing cell
lysis. Vaginal secretions are also slightly
acidic. Spermine and zinc in semen destroy
some pathogens.
6. The stomach is a formidable obstacle
insofar as it secretes hydrochloric acid
(0.9 < pH < 3.0, very acidic) and proteindigesting enzymes that kill many
pathogens. The stomach can even destroy
drugs and other chemicals.
Inflammatory Response
• The inflammatory response occurs when
tissues are injured by bacteria, trauma,
toxins, heat, or any other cause.
• Chemicals, such as histamine, bradykinin
and serotonin, are released by damaged
tissue. These chemicals cause blood vessels
to leak fluid into the tissues, causing swelling.
This helps isolate the foreign substance from
further contact with body tissues.
• Interleukin is one of the most important
chemicals. It stimulates lymphocytes and
other white blood cells into action.
It also promotes fever by resetting the body's
thermostat located in the hypothalamus.
• The chemicals also attract white blood cells
that "eat" microorganisms and dead or
damaged cells. The process in which these
white blood cells surround, engulf, and
destroy foreign substances is called
phagocytosis, and the cells are called
phagocytes. Phagocytes eventually die. Pus
is formed from a collection of dead tissue,
dead bacteria, and live and dead phagocytes.
Summary of the inflammatory response
• Entry of microbes (i.e. bacteria).
• Vasodilation of the microcirculation (small blood
vessels) resulting in increased blood flow.
• An increase in vascular permeability to protein.
• Filtration of fluid into the tissue which leads to
swelling.
• Neutrophils (a type of white blood cell) and later
monocytes (another type of white blood cell)
migrate from the blood vessels into the tissues.
• Phagocytosis and destruction of the microbes.
• Tissue repair.
Complement Response
• Complement is a group of serum proteins
which work with (complement) antibody
activity to eliminate pathogens.
• Complement is NOT antigen-specific and it is
activated immediately in the presence of
pathogen, so it is considered part of innate
immunity.
• However, antibody activates some
complement proteins, so complement
activation is also part of humoral immunity.
• Complement proteins are present in the
circulation as inactive molecules.
• Several complement proteins are proenzymes. When activated, they become
proteases that cut peptide bonds in other
complement proteins to activate them in turn.
• Since each activated protease can activate
many substrate molecules, the initial
activation is rapidly amplified to produce
millions of activated molecules (a cascade).
Complement activation stimulates several
antimicrobial activities:
1. One activity is the formation of a membrane
attack complex (MAC), which inserts into lipid
membranes of bacteria or eukaryotic cells and
causes osmotic lysis.
2. Complement fragments called opsonins
adhere to microorganisms and promote
phagocytosis, and activation of macrophage
and neutrophil killing mechanisms.
3. Complement fragments called anaphylatoxins
promote an inflammatory response by binding
to complement receptors on mast cells and
triggering release of histamine, which increases
blood vessel permeability and smooth muscle
contraction.
A little quiz
1. Complement
a. is a group of active proteolytic enzymes
found in serum.
b. is secreted by macrophages and
hepatocytes in response to antigen binding.
c. participates in both innate and adaptive
immune responses.
d. prevents lysis of virus-infected cells.
e. all of the above statements about
complement are true.
2. Complement is involved in all of the following
except
a. attraction of neutrophils to an infection site.
b. increased presence of serum proteins in the
infected tissues.
c. lysis of bacteria in the absence of specific
antibodies.
d. opsonization of microorganisms for
phagocytosis.
e. sensitization of T cells to antigen
3. Complement is
a. activated by binding to specific complement
receptors.
b. antigen-specific.
c. a potent promoter of virus entry into host
cells.
d. a series of intracellular proteins which work
with antibody to eliminate endogenous
antigen.
e. present in the circulation in an inactive form.
Answers are at the end of the pp
SPECIFIC IMMUNITY
• Specific immunity, also known as
adaptive immunity, normally comes into
play when innate or non-specific
immunity can't handle the problem.
• This form of immunity is a more recent
evolutionary development than innate
• It is distinguished by its specificity for
an invading organism and for its ability
to remember (anamnesis) an encounter
so that the second time the same
organism or invader is encountered a
more rapid and intense response can
occur.
• There are two types of specific immune
responses: humoral and cell mediated.
• Humoral immunity is mediated by B
Lymphocytes that produce antibodies (IgG,
IgM, IgA, IgE) and is especially important in
combating acute bacterial infections.
• Cellular immunity is mediated by T
Lymphocytes which are processed in the
thymus. Cellular immunity is especially
important in combating intracellular
organisms such as TB, performing tumor
surveillance, mediating transplant rejection,
and fighting fungal and viral infections.
Cell mediated immunity
T Cells: have two major roles in immune
defense.
1. Regulatory T cells are essential for directing
and organizing the response of different types
of immune cells.
– Helper T cells (aka CD4 cells)
• Alert B cells to start making antibodies;
• Activate other T cells
• Activate macrophages
• Influence which type of antibody is produced.
2. Killer T Cells destroy infected cells
Killer T cells (aka CD8 cells):
– attack and destroy infected cells by lysis.
– The killer T cells are also called cytotoxic T
cells.
Activation of helper T cells
After it engulfs and processes
an antigen, the macrophage
displays the antigen fragments
combined with a Class II MHC
protein on the macrophage cell
surface. The antigen-protein
combination attracts a helper T
cell, and promotes its
activation.
Activation of killer T cells
After a macrophage engulfs and processes an antigen, the
macrophage displays the antigen fragments combined with a
Class I MHC protein on the macrophage cell surface. A
receptor on a circulating, resting killer T cell recognizes the
antigen-protein complex and binds to it. The binding process
and a helper T cell activate the killer T cell so that it can attack
and destroy the diseased cell.
Humoral immunity
• B cells produce antibodies that circulate in
the blood and lymph streams and attach to
foreign antigens to mark them for
destruction by other immune cells.
• B cells are part of what is known as
antibody-mediated or humoral immunity
Activation of B cells to make antibody
A B cell uses one of its
receptors to bind to its
matching antigen, which the B
cell engulfs and processes.
The B cell then displays a
piece of the antigen, bound to
a Class II MHC protein, on the
cell surface. This whole
complex then binds to an
activated helper T cell. This
binding process stimulates the
transformation of the B cell
into an antibody-secreting
plasma cell.
Antibody formation by B cells
B cells become plasma cells,
which produce antibodies
when a foreign antigen
triggers the immune response.
Go to this site
• http://www.biologymad.com/
and click on Immunology
The explanations are clear, concise, and the
illustrations are great!
Answers to “A little quiz”
1. C
2. E
3. E