Download Unit 2.2.2 – Health and Disease Immunity

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Unit 2.2.2 Health and disease
Unit 2.2.2 – Health and Disease
Health and Disease
1948 World Health Assembly:
“Health is a state of complete physical, mental, and social well-being and not merely the
absence of disease or infirmity.”
Think about this for a moment. Many people associate health with not being physically ill.
But in fact, healthcare professionals play a much wider role in ensuring people have a good
quality of life.
Disease:
Very difficult to define. Here are some thoughts:
“An abnormal condition of the body or mind that causes discomfort or dysfunction; distinct
from injury insofar as the latter is usually instantaneously acquired.”
“An alteration of a living body that impairs its functioning”
“A condition of being sick from a particular cause”
Immunity
What is immunity?
In order to remain healthy and able to function properly, the body must protect itself from
disease-causing organisms, known as pathogens. The study of the mechanisms used to do
this is known as immunology.
The human body is well adapted to prevent pathogens (mainly bacteria, fungi and viruses)
from entering in the first place. There are several ways in which this is achieved:
-
the skin forms a protective barrier against most pathogens
o if the skin is cut, blood clotting helps to seal the wound
-
the mucus membranes of the nose and respiratory tract filter air using small hairs
known as cilia and trap pathogens in the mucus for destruction by phagocytes.
-
anti-bacterial enzymes in saliva, sweat and tears help destroy pathogens
-
the stomach acid (pH ≈ 2) kills most pathogens, which may enter via food or water
If, however, these mechanisms fail and pathogens do enter the body tissue or blood, an
immune system is necessary to attack and destroy these pathogens and so prevent serious
disease or even death. The immune system uses several specialised types of white blood
cells: the most important being phagocytes and lymphocytes.
SICM Tuition
Unit 2.2.2 Health and disease
Phagocytes
Phagocytes are white blood cells, which can engulf and digest pathogens (as well as other
foreign materials and dead or infected cells). They are non-specific in their actions and so
will deal with any type of foreign material, which they come across. They have a distinctive
appearance with a lobed nucleus and a grainy cytoplasm.
Phagocytes are mainly found in the blood and the lymphatic system, especially in the
lymph nodes. They are also capable of squeezing through tiny gaps between cells in the
walls of capillaries and entering the tissue fluid, which surrounds every cell.
granular cytoplasm
lobed nucleus
Phagocytosis – phagocytes engulfing/digesting bacteria
phagocyte
(nucleus not shown)
bacterium giving
out chemical
lysosomes
(containing powerful
digestive enzymes)
vacuole forming
bacterium is trapped and
some lysosomes move
towards the vacuole and
fuse with it.
the bacterium is broken
down and passes into
the cytoplasm of the
phagocyte
Some definitions
antigen
-
foreign protein (on the surface of the membrane of a virus or bacteria),
which stimulates the production of a specific antibody
antibody
-
chemical substance produced by a B-lymphocyte in response to
exposure to a specific antigen.
Antibodies
They are typically made of basic structural units—
each with two large heavy chains and two small light
chains—to form, for example, monomers with one
unit, dimers with two units or pentamers with five
units.
Although the general structure of all antibodies is
very similar, a small region at the tip of the protein is
extremely variable, allowing millions of antibodies
with slightly different tip structures to exist. This
region is known as the hypervariable region. Each of
these variants can bind to a different target, known as
an antigen. This huge diversity of antibodies allows
the immune system to recognize an equally wide
diversity of antigens.
SICM Tuition
Unit 2.2.2 Health and disease
Lymphocytes
Lymphocytes are white blood cells, which are
involved in very specific immune responses
against pathogens. They do not engulf and
digest pathogens, but use other, more
complicated processes to destroy them. Like
phagocytes, lymphocytes circulate in the
blood and lymph fluid and are also found
within the lymph nodes. They have a large
rounded nucleus, which almost fills the cell
and a small amount of non-grainy cytoplasm.
There are two main types of lymphocytes
known
as
B-lymphocytes
and
TDifferent shaped antibodies
lymphocytes. Both of these lymphocytes
respond to the presence of specific types of foreign material in the body and bring about
actions to remove these. Although they actually work in very different ways, the starting
point for this is the recognition of the antigen.
With regard to pathogens, antigens are surface protein molecules, which trigger an immune
response.
non-granular cytoplasm
regularly-shaped nucleus
The Immune response
B and T-lymphocytes recognise specific pathogens that have invaded the body and set in
motion mechanisms to destroy them: thus protecting the body from harm. This is known
as the immune response.
The function of B-lymphocytes
B-lymphocytes are involved in the production of antibodies in response to antigens, which
is called humoral immunity.
On the surface of the membrane of B-lymphocytes are a number of specific antigen
receptors. These are sites to which antigens on the surface of pathogens may become
attached, leading to a sequence of events in which antibodies are produced to prevent the
pathogens from causing harm.
There are many thousands of specific types of B-lymphocytes and each is capable of
recognising only one specific antigen from a pathogen. (For example, only one type of Blymphocyte will attach to the bacterium causing Tuberculosis, another one for Cholera)
This specificity is due to the slight differences in the shape of the antigen receptor on each
B-lymphocyte.
SICM Tuition
Unit 2.2.2 Health and disease
Sequence of events
-
Pathogens enter the body. This may be via droplets in the air, by a vector (e.g.
mosquito), via food, water or body fluids such as saliva, blood or semen.
-
Antigens on the surface of the pathogen come into contact with their specific
antigen receptor on a B-lymphocyte.
-
The binding of the antigen to the antigen receptor activates the B-lymphocyte and
causes it to divide producing a clone of identical B-lymphocytes. This is often
known as a clonal explosion.
-
Most of the B-lymphocytes turn into plasma cells and the rest turn into memory
cells.
-
The plasma cells begin to secrete antibodies against the pathogen concerned.
Antibody molecules are secreted at a very high rate: up to 30 000 molecules per
second.
-
The antibodies attach to the antigens on the pathogen and lead to the destruction of
the pathogen.
-
Once the pathogens have been destroyed, the plasma cells eventually die and
antibodies stop being secreted. But, the memory cells remain in the lymph nodes and
the circulation in case of further infection with the same pathogen.
The Immune response
B-cell replicated
many times:
clonal explosion
enlargement of B-cells to form
plasma cells. These produce
antibodies.
pathogen with
antigens
antibodies
B-cell is activated
due to antigen
memory B-cells: enable rapid
response following subsequent
exposure to the same antigen
T-lymphocytes
T-lymphocytes recognise foreign antigens and help in antibody production. Others bind to
the pathogen and present the antigen to the B-lymphocytes.