The 3rd Line of Defense – The Immune System
Introducing… Leukocytes - White Blood Cells (WBCs)
In the bone marrow, stem cells are made.
Stem cells can turn into all kinds of cells.
Some of them turn into erythrocytes - red blood cells
RBCs (which carry oxygen around our body) - and
some of them turn into WBCs (which are part
of our 3rd line of defense – the immune system).
Platelets are also produced in the bone marrow and
these allow our blood to clot if we get a cut (but this is
part of the 2nd line of defense, the inflammation response).
There are many types of WBCs. The picture on
the left shows the main types.
We are focusing only on the macrophages, B
Cells & T Cells and plasma cells.
These WBCs can travel around in the blood
stream to the places in the body where they are
needed. T Cells mature in the thymus,
B Cells mainly mature in the bone marrow.
When monocytes get to the tissues, they
transform into macrophages.
Macrophages are part of our instant,
or innate immune response.
Macrophages are a type of phagocyte –
A cell that engulfs and absorbs waste material,
harmful microorganisms, or other foreign
bodies in the bloodstream and tissues.
That means, they can engulf invaders in the body.
Macrophages are not specific (eg, they don`t only engulf one type of
bacterial cell), they are generalists and can engulf
all types of infected or dead cells.
A part of the engulfed invader that is unique to that invader is called
an antigen. Macrophages then “present” (show) the antigen to other
WBCs which cause other immune cells to start up a wider and more
specific immune response attack on the invader (bacteria or virus).
The later, adaptive immune response – is in two parts;
1. Cell-mediated immune response
2. Antibody-mediated immune responses
Macrophages presenting the antigen to the immune system is the beginning of both responses
Lymphocytes account for approximately 25 percent of WBCs – they are colorless cells found in the blood
and lymph system.
There are two types; B Cells (B lymphocytes) and T Cells (T lymphocytes).
1. T Cells are involved in the Cell-mediated immune response.
There are 2 main types of T Cell
- Helper T Cells
- Killer T Cells (also known as Cytotoxic T Cells or Natural Killer T Cells – NK Cells)
Helper T Cells
Helper T cells are the major
driving force and regulators of the
immune defense. Their task is to
activate B cells and killer T cells.
However, the helper T cells
themselves must be activated.
This happens when a macrophage,
which has eaten an invader,
travels to the nearest lymph node
to present information about the
captured pathogen (ie, present the
antigen). When a helper T cell
recognizes the antigen, the T cell
is activated. Once activated, helper
T cells start to divide and to
produce proteins that activate B
and T cells as well as other
immune cells.
Killer T Cells
The killer T cell is specialized in attacking
cells of the body infected by viruses and
and bacteria. It can also attack cancer cells.
The killer T cell has receptors that are used
to search each cell that it meets. If a cell is
infected, it is swiftly killed. Infected cells
are recognized because tiny traces of the
intruder, antigen, can be found on their
surface.
The immune system remembers invaders!
When a person has been exposed to a particular invader before, such as a strain of influenza, the body is
able to remember it because it produced memory cells after the first attack. Memory Helper T Cells and
memory killer T Cells are two types of memory cells; they can respond faster if there is an attack by the
same invader in the future because they already know how to recognize the antigen when it is presented
by a macrophage.
2. B Cells are involved in the Humoral (antibody-mediated) response.
You can see from the diagram below, that when there was the first attack (1st exposure) of this strain of
influenza, the helper T cells activated a B Cell. B Cells are the main form of memory for the immune
system. They can mount a specific attack on the invader in the form of antibodies. Antibodies can be
produced at around 2000 antibodies per second, and then travel in the blood and tissues, find the invaders
and neutralize them so that macrophages can gobble them up and then the debri is removed from the body
through the lymph system.
When the B cell for this strain of virus was created by the body, it produced clones of itself (many copies) in
the form of plasma cells – these are the cells that then produce the antibodies; they are antibody making
factories. A memory B cell was also formed, so if the same strain of influenza tries to attack a person a
second time (2nd exposure), the persons memory B cells will respond very quickly to produce plasma cells
(antibody making factories), which will produce lots of antibody quickly to neutralize the invader.
So the cell-mediated response is less important in a second attack, though it is still the first of the two
immune responses to occur, and the invaders are wiped out before the infected human feels any
symptoms. The body has achieved immunity against the invader. This is the basis of immunization
(vaccination).
Sources of images and information:
http://www.abc.net.au/health/library/stories/2004/10/18/1830091.htm
http://www.uic.edu/classes/bios/bios100/lectures/macrophage.jpg
http://www.nobelprize.org/educational/medicine/immunity/immune-detail.html
http://www.uic.edu/classes/bios/bios100/lectures/immune.htm
Questions
1. Define the following:
(a) Phagocytosis:
(b) Innate:
(c) Antigen:
(d) Antibody:
(e) Engulf:
(f) Secrete:
The diagram below shows you how your body can defend itself from invading pathogens.
This diagram applies to the next four questions.
11.
Identify how the pathogen entered the body:
___________________________
12.
Where would you normally find an antigen?
___________________________
13.
Name the cell that produces plasma cells
___________________________
14.
Which two components remain in the blood for future protection from the same
pathogen? ___________________________ ___________________________
15. What is another name for a B cell?
16. What is another name for a T cell?
17. Where would the killer T cells fit into the diagram above?
Maternal, natural vs aquired immunity