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Defence against infectious diseases

Topic 6
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3

Biology SL

Urethra

Tube that carries urine from the bladder to

Vagina

The reproductive tract leading from the

the outside

uterus to the outside

Blood clotting
➢ When small blood vessels are broken, blood escapes from the closed circulatory
system and so pathogens have a way to gain access to the body, as these blood
vessels are often in the skin
○

The body responds creating a clot that seals the damaged blood vessel
preventing excessive blood loss and helping prevent pathogens from
entering the body

➢ Circulating in the blood plasma are a variety of molecules called plasma proteins
some of which are involved in clotting
○

Two of them are prothrombin and fibrinogen

➢ Also circulating in the bloodstream are cell fragments known as platelets
○

They form in the bone marrow along with erythrocytes and leukocytes

○

They don’t remain as entire cells but instead, one very large cell breaks
down into many fragments and each of the fragments becomes a platelet

○

They don’t have a nucleus but they have a short cellular life span of about
8-10 days

➢ Steps
○

The damaged cells of the blood vessel release chemicals that stimulate
platelets to adhere to the damaged area

○

The damaged tissue and platelets release chemicals called clotting factors
that convert prothrombin into thrombin
■

Thrombin is an active enzyme that catalyses the conversion of
soluble fibrinogen into the relatively insoluble fibrin

○

Fibrin, a fibrous protein, forms a mesh-like network that helps to stabilize
the platelet plug

○

More and more cellular debris becomes trapped in the fibrin mesh and soon
a stable clot has formed

2

Defence against infectious diseases

Topic 6
...
, it’s known as the secondary, tertiary, etc
...
3

Biology SL

Antibodies

➢ Antibodies are Y-shaped proteins that are produced by the body and that can bind
to the surface of pathogens
➢ Each type of antibody is different, because each type has been produced in response
to a different pathogen
➢ Each pathogen is made up of either cells with cell membranes or in case of a virus, a
protein coat called capsid
○

The cellular invaders have proteins that are embedded in their outer surface
called antigens
■

Non-specific proteins = antigens

➢ Each antibody at the end of the forks of the Y is a binding site
○

The binding site is where an antibody attaches itself to an antigen, meaning
that it attaches to the pathogen

➢ The leukocytes that produce the antibodies are a type of cell called plasma cells
○

Each of has has many different types of antibody-producing plasma cells
and each type can produce only one type of antibody

○

Each cell produces a relatively small number of antibodies but our immune
response has a way of producing many when they are needed

➢ Steps
○

A specific antigen type/pathogen is detected and identified by a white blood
cell

○

A specific plasma cell, called lymphocyte, that can produce an antibody that
will bind to the antigen is identified

○

The specific plasma cell type clones itself by mitosis to increase rapidly in
number

○

The specific plasma cells begin to produce antibodies

○

The antibodies circulate in the bloodstream and eventually find their
antigen match

○

Using various mechanisms, the antibodies help eliminate the pathogen

○

Some of the plasma cells remain in the bloodstream and provide immunity
against a second infection by the same pathogen
■

○

These long-lived cells are called memory cells

Memory plasma cells of this type respond quickly if the same antigen is
encountered again

➢ Vaccines are weakened or non-pathogenic forms of pathogens that cause a primary
immune response producing the same lymphocytes as the actual disease
4

Defence against infectious diseases

Topic 6
...
3

Biology SL

➢ Antibiotics are chemicals that take advantage of the differences between
prokaryotic and eukaryotic cells and selectively block some of the biochemistry
needed by bacteria
○

They have no effect on animal cells

➢ Many categories of antibiotics
○

One may block protein synthesis in bacteria

○

Or may inhibit the production of a new cell wall by bacteria blocking the
ability to grow and divide

➢ Antibiotics have no effect on viruses since they have no metabolism of their own
○

Antibiotics often target specific metabolic pathways

○

Since viruses use our own cells against us, a medicine against the virus
would also harm our body cells

Penicillin
➢ One of the most famous antibiotics, produced by the fungus Penicillium
➢

The effect was first discovered by Alexander Fleming in 1928
○

A petri dish where bacteria grew were inhibited by penicillin and no bacteria
continued to exist

➢ Fleming was unable to isolate the chemical produced by the fungus, but around
1940 Ernst Chain and Howard Florey were able to do it and tested the chemical on
mice
○

They infected 8 mice with deadly bacteria
...
The penicillin was given in high dose to an
ill person who nearly died

Resistance
➢ A growing problem today is the resistance of some bacteria to antibiotics
○

In a bacterial population there are millions of individuals

○

Bacteria reproduce rapidly

○

Mutations occur and these mutations can lead to resistance to antibiotics
■

A bacterium which is resistant to an antibiotic due to mutation can
reproduce fast and build a colony of resistant bacteria

➢ The long-term use and overuse of antibiotics has led many pathogens to be
resistant to nearly all of the antibiotics today
➢ MRSA (pronounced mersa) are strains of Staphylococcus aureus, a pathogen which
has developed a resistance to many types of antibiotics
6



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