Download Lecture #23 - Suraj @ LUMS

Lecture 23
Immune System
Introduction
• A human or animal must defend itself against
multitude of different pathogens including viruses,
bacteria, fungi, and protozoan.
• Prevent disruption to homeostasis.
• There are plenty of effector mechanisms capable
of defending the body against such agents.
• They can be soluble molecules or cells.
Definitions
• When an organism is threatened by microorganisms,
viruses, or cancer cells, the immune response acts to
provide protection.
• Normally, the immune system does not mount a response
against self. This lack of an immune response is called
tolerance.
• In some cases, the immune system does mount an immune
response against self. If an error is made, and an immune
response is made against self, tolerance to self is lost. This
condition is called autoimmunity (from Greek, "selfimmunity"). Examples of autoimmune diseases in humans
are: asthma, lupus, and arthritis.
Immunity
• The body's capability to repel foreign substances and cells.
• The nonspecific (innate) responses are the first line of
defense - block the entry and spread of disease-causing
agents.
• Highly specific (adaptive) responses are the second line of
defense and are tailored to an individual threat.
• Antibody-mediated and cell-mediated responses are two
types of specific response.
• The immune system is associated with defense against
disease-causing agents, problems in transplants and blood
transfusions, and diseases resulting from over-reaction
(autoimmune, allergies) and under-reaction (AIDS).
Non-specific Vs Specific Mechanisms
• Active defensive mechanisms function to avoid and limit
damage and disease or to facilitate repair and recovery.
• Some active mechanisms are triggered whenever tissue
damage or disease occurs and so are called non-specific.
• When they operate together in a co-ordinated way the
result is called inflammation.
• Non-specific mechanisms provide for innate (inborn)
immunity, i.e. they protect us effectively without any
previous exposure to the agents of damage or disease (like
viruses and bacteria).
• Specific mechanism provide for acquired immunity, i.e.
they become effective only after the body has been
exposed to specific agents of disease.
General Defenses
• Physical barriers are the first line of defense.
• Barriers to entry are the skin and mucous
membranes.
• The skin is a passive barrier to infectious agents
such as bacteria and viruses.
• Tears and saliva secrete enzymes that breakdown
bacterial cell walls. Skin glands secrete chemicals
that retard the growth of bacteria.
• Mucus membranes lining the respiratory,
digestive, urinary, and reproductive tracts secrete
mucus that forms another barrier.
Non Specific Defense
The inflammatory response.
1.
2.
3.
Damaged cells release chemical signals such as histamine
that increase capillary blood flow into the affected area
(causing the areas to become heated and reddened).
White Blood Cells can move out of the blood vessels to
the site of infection.
Phagocytes are a type of White Blood Cell that will
recognize and engulf bacteria and other foreign
substances, including cell debris left behind by.
Macrophages are white blood cells that function BOTH
in non-specific and specific defenses. They attack and
engulf viral-infected cells in a non-specific manner, but
then PRESENT parts of the foreign virus particle to the
immune system in order to trigger an immune response.
Inflammation
• The basic symptoms of inflammation are:
-redness,
-swelling,
-heat,
-pain.
Inflammatory Response - 1
Three major events occur during this response :
1. An increased blood supply to the tissue ''in danger''. It is
performed by vasodilation.
2. Increased capillary permeability caused by retraction of the
endothelial cells. Enables larger molecules than usual to
escape from the capillaries, and thus allows the soluble
mediators of immunity to reach the site of inflammation.
3. Leukocytes migrate out of the capillaries into the
surrounding tissues. In the earliest stages of inflammation,
neutrophils are particularly prevalent, but later monocytes
and lymphocytes also migrate towards the site of infection.
Inflammatory Response - 2
• Neutrophils, the most active and phagocytic of the white blood cells,
become sticky and begin to adhere to the inside of the vessel wall.
• Adherence slows the cells down, making them "roll" on the inside of
the vessel.
• The neutrophils then become superadherent and squeeze out between
endothelial cells that line the vessel, a phenomenon called
"diapedesis".
• From there, the white cells must FIND, EAT, and KILL foreign
microbes.
• CHEMOTAXIS ... how white cells FIND the bad guys
• PHAGOCYTOSIS ... how white cells EAT microbes
• OXIDATIVE BURST ... how white cells KILL bacteria
Foreign Bodies
• A n t i g e n s ( Ag )- term coined long
ago…because they are antibody generating,
usually large, foreign molecule (hopefully), can be
proteins and carbohydrates.
• E p i t o p e s - specific portion of the antigen that
is recognized by an antibody.
• H a p t e n s - are small organic molecules that are
bound by antibodies, but do not illicit immune
response, are antigenic when combined with a
larger molecule.