Download First line of nonspecific defenses Second line of nonspecific

We survive because our body’s immune system defends us against pathogens
(disease-causing agents). The immune system consists of cells and tissues found throughout the body. The body
uses both nonspecific and specific defense mechanisms to prevent infection and to detect and destroy pathogens.
First line of nonspecific defenses
The body’s surface defenses are nonspecific, meaning they do not target specific pathogens.
Skin is the first of our immune system’s nonspecific defenses against pathogens.
o Skin acts as a nearly impenetrable barrier to invading pathogens, keeping them outside the body.
o This barrier is reinforced with chemical weapons – oil and sweat!
o Oil and sweat make the skin’s surface acidic, inhibiting the growth of many pathogens and sweat contains
the enzyme lysozyme which digests bacterial cell walls.
Internal surfaces of the body through which pathogens can pass are covered by mucous membranes.
o Mucous membranes are layers of epithelial tissue that produce sticky, viscous fluid called mucus.
o Mucous membranes line the digestive system, nasal passages, lungs, respiratory passages, and the
reproductive tract.
Skin and mucous membranes work to prevent any pathogens from entering the body; nevertheless, sometimes
these defenses are penetrated.
When invaders reach deeper tissue, a second line of nonspecific defenses takes over.
Second line of nonspecific defenses
When the body is invaded, four important non-specific defenses take action:
1. The Inflammatory Response
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
Caused by injury or local infection, like a cut or a scrape
Is a series of events that suppress infection and speed recovery
Imagine that a splinter has punctured your finger, creating an entrance for pathogens.
Infected or injured cells in your finger release chemicals, including histamine.
Histamine causes local blood vessels to dilate, increasing blood flow to the area.
Increased blood flow brings white blood cells to the infection site, where they can attack pathogens.
This also causes swelling and redness in the infected area.
The whitish liquid, or pus, associated with some infections contains white blood cells, dead cells and
dead pathogens.
2. The Temperature Response
ƒ
ƒ
ƒ
ƒ
ƒ
When the body begins its fight against pathogens, body temperature increases several degrees above
the normal value of about 37°C (99°F).
This higher temperature is called a fever, and it is a common symptom of illness that shows the body
is responding to an infection.
Fever is helpful because many pathogenic bacteria do not grow well at high temperatures.
But, very high fever is dangerous because extreme heat can destroy important cellular enzymes.
Temperatures greater than 39°C (103°F) are considered dangerous and those greater than 41°C
(105°C) can be fatal.
Biology 20
The Immune System
Page 1 of 7
Source: Johnson, Raven. 2001. Biology: Principles and Explorations. Holt, Rinehart and Winston. Pages 931 - 937
3. Proteins that kill or inhibit pathogens
ƒ
ƒ
Certain proteins circulate in the blood and become active when they encounter certain pathogens.
♦ Then they form a membrane attack complex (a ring-shaped structure that ruptures the cell
membrane of pathogens).
Another nonspecific defense is interferon (a protein released by cells infected with viruses).
♦ Causes nearby cells to produce an enzyme that prevents viruses from making proteins and RNA.
4. White Blood Cells
ƒ
ƒ
ƒ
The most important counterattacks in the second line of nonspecific defenses are carried out by three
types of white blood cells: Neutrophils, Macrophages and Natural killer cells.
These cells patrol the bloodstream, wait within the tissues for pathogens, and then attack the
pathogens.
Each kind of cell uses a different mechanism to kill pathogens:
Neutrophils
♦ Is a WBC that engulfs and destroys pathogens.
♦ Most abundant type of WBC
♦ Sacrifice themselves to defend the body
♦ Engulf bacteria and then release chemicals that kill the bacteria and themselves
Macrophages
♦ Ingest and kill pathogens they encounter
♦ Clear dead cells and other debris from the body
♦ Most travel through the body in blood, lymph, and fluid between cells
♦ Concentrated in particular organs, especially the spleen and lungs.
Natural Killer Cells
♦ Large WBC that attacks cells infected with pathogens
♦ Destroy an infected cell by puncturing its cell membrane
♦ Water then rushes into the infected cell, causing the cell to swell and burst
♦ One of the body’s best defenses against cancer – can detect cancer cells before a tumor can
develop
Biology 20
The Immune System
Page 2 of 7
Source: Johnson, Raven. 2001. Biology: Principles and Explorations. Holt, Rinehart and Winston. Pages 931 - 937
Third line of defense – Specific immune response
What happens when pathogens occasionally overwhelm your body’s nonspecific defenses? Pathogens that
have survived the first and second lines of nonspecific defences still face a third line of specific defenses –
the immune response.
The immune response consists of an army of individual cells that rush throughout the body to combat specific
invading pathogens.
Cells Involved in the Immune Response
WBC are produced in bone marrow and circulate in blood and lymph.
Of the 100 trillion cells in your body, about 2 trillion are WBC.
Four main kinds of WBC participate in the immune response: macrophages, cytotoxic T cells, B cells, and
helper T cells.
Each kind of cell has a different function:
Macrophages – consume pathogens and infected cells
Cytotoxic T cells – attack and kill infected cells
B cells – activate both cytotoxic T cells and B cells
Helper T cells – activate both cytotoxic T cells and B cells
These four kinds of WBC interact to remove pathogens from the body.
What’s the big deal about viruses anyways??
Well, first of all, viruses KILL their host cell. In viral infections, many cells in the body are
infected and then destroyed. The fact that each infected cell produces many new virus
particles, each of which can go on to infect other cells, just amplifies the damaging effects of
viruses!!
Biology 20
The Immune System
Page 3 of 7
Source: Johnson, Raven. 2001. Biology: Principles and Explorations. Holt, Rinehart and Winston. Pages 931 - 937
Recognizing Invaders
Imagine that you have just come down with the flu. You have inhaled the influenza viruses, but they were not
trapped by mucus in the respiratory tract. The viruses have begun to infect and kill your cells.
At this point, macrophages begin to engulf and destroy the viruses.
An infected body cell will display antigens of an invader on its surface.
o An antigen is a substance that
triggers an immune response.
Antigens typically include proteins and other components of
viruses or pathogen cells present on the cell surface.
WBCs of the immune system are covered with receptor
proteins that respond to infection by binding to specific
antigens on the surfaces of the infecting microbes.
These receptors recognize and bind to antigens that match
their particular shape.
Immune response has two main parts
Two distinct processes work together in an immune response.
o B cell response - a passive defense that aids the removal of pathogens from the body.
o T cell response – an active, cell-mediated defense that involves the destruction of pathogens by cytotoxic
T cells.
Both the T cell response and B cell response are regulated by helper T cells.
Step 1: When a virus infects body cells, the infected cells display the viral antigen on their surface.
Step 2:
Macrophages engulf the virus and display the viral antigen on their surface.
Step 3:
Receptor proteins on helper T cells bind to the viral antigen displayed by the macrophages. The
macrophages release a protein called interleukin-1.
Step 4:
Interleukin-1 activates helper T cells, but helper T cells do not attack pathogens directly. Instead,
helper T-cells activate cytotoxic T cells and B cells. Stimulation by interleukin-1 causes helper T
cells to release interleukin-2. Interleukin-2 stimulates further division of helper T cells and cytotoxic
T cells, amplifying the body’s response to the infection.
Step 5:
Interleukin-2 released by helper T cells also activates B cells. When activated by interleukin-2, B
cells divide and develop into plasma cells. Plasma cells are cells that release special defensive
proteins into the blood. These specialized proteins are called antibodies. An antibody is a Yshaped molecule that is produced by plasma cells upon exposure to a specific antigen and that can
bind to that antigen.
Step 6:
Plasma cells divide repeatedly and make large amounts of antibodies. Plasma cells either release
antibodies into the bloodstream or attach them directly to the virus. Antibodies bind to the viral
antigen on the virus and on infected cells. Antibodies mark the virus and infected cells for
destruction.
Biology 20
The Immune System
Page 4 of 7
Source: Johnson, Raven. 2001. Biology: Principles and Explorations. Holt, Rinehart and Winston. Pages 931 - 937
Step 7:
When a plasma cell encounters a virus with an antigen that matches its antibodies, it binds to the
virus. This causes other viruses to stick together, forming a clump that can be easily identified and
destroyed by macrophages.
Step 8:
With the help of antibodies and plasma cells, cytotoxic T cells destroy infected cells by puncturing
the cell membrane of the infected cells.
How do cytotoxic T cells recognize antigens? Your body makes millions of different T
cells, each with receptor proteins that bind to a specific antigen. Receptor proteins on cytotoxic T
cells bind to the viral antigen displayed by infected cells. For example, any of your body’s cells that
bear traces of an influenza virus will be destroyed by cytotoxic T cells with receptor proteins that
bind to the antigen of that virus.
Biology 20
The Immune System
Page 5 of 7
Source: Johnson, Raven. 2001. Biology: Principles and Explorations. Holt, Rinehart and Winston. Pages 931 - 937
Immune
Type of Cell
Macrophage
Neutrophil
Natural Killer Cell
Helper T Cell
Cytotoxic T Cell
B Cell
Plasma Cell
Memory Cell
Cells –
Function
Ingests and kills pathogens
Engulfs and destroys pathogens
Punctures infected cells
Activates cytotoxic T and B cells
Punctures infected cells
Labels invaders for destruction by
macrophages
Releases antibodies
Protects against defeated pathogens
An
Overview
Location in the Body
Spleen, lungs, blood, lymph, interstitial fluids
Bloodstream, infection sites
Infected cells
Bloodstream, infection sites
Infection sites
Infection sites
Bloodstream
Bloodstream
Check this out:
http://www.lupus.ab.ca/viewpage.asp?p=resources-flash
Biology 20
The Immune System
Page 6 of 7
Source: Johnson, Raven. 2001. Biology: Principles and Explorations. Holt, Rinehart and Winston. Pages 931 - 937
Biology 20
The Immune System
Page 7 of 7
Source: Johnson, Raven. 2001. Biology: Principles and Explorations. Holt, Rinehart and Winston. Pages 931 - 937