6.3 Defence against infectious diseases

6.3 Defence against infectious diseases

Lesson one: the barrier system

Essential questions: If there are bacteria everywhere, why do we only get sick sometimes? What happens when we get sick, and what happens when we recover? What happens when blood clots?

Introduction: Watch this clip from a movie on the pioneers of penicillin. Prepare a personal response; what role did each scientist actually play? Alexander Flemming; Norman Heatley, Howard Florey, Edward Chain.

Q. Where Florey and Chains experiments ethical by modern standards?

A. Yes because they saved lives especially in WW2. No because they only tested penicilin on rats, and then a small number of humans; and then released it.

The barrier system is a number of defence mechanisms designed to PREVENT infection by pathogens. A pathogen is a disease-causing agent eg. Ebola virus. An infection means that a viable population of a pathogen has been established inside the body.

The barrier system is the first line of defence.



image credit: biology aid

image credit: biology aid


Discuss: How can you aid your body in maintaining a more effective barrier system?

 The role of Mucus:

Certain parts of the body like the respiratory passages are lined by thin layers of skin called ‘mucus membranes’.  Examples include the openings to the reproductive tracts (head of penis and vagina). In order to protect them, mucus is produced which:

  • Pathogens and foreign particles stick to
  • Can be removed (eg. sneezing) or in the case of the lining of the nose, swallowed.
  • Has anti-septic properties.

Repairing the skin layer

If the skin layer is cut, clotting occurs. Clotting is a mechanism that prevents:

  • Entry of pathogens into the blood stream
  • Further bleeding

A blood clot includes a mesh of protein fibres called ‘fibrin’, which traps red blood cells. It is platelets, a kind of  blood cell, which are responsible for this mechanism.

Blood clotting

image credit: biology mosaic

image credit: biology mosaic


1) Skin is cut

2) Platelets aggregate at cut, forming a temporary plug

3) Platelets release clotting factors

4) Clotting factors cause soluble fibrinogen to become insoluble fibrin

5) A clot begins to form.





Link to last topic: A coronary thrombosis is a blood clot in the coronary arteries. This has very serious consequences, as it would effectively block or at least significantly reduce the supply of blood to the cardiac muscle, resulting in oxygen starvation in one part of the heart.

The result – Fibrillation, or irregular and uncoordinated contractions of the cardiac muscle.

10 minute socractic style discussion:

  1. Q) What actually is blood clotting?
  2. Q) Can you clarify how blood clotting occurs?
  3. Q) What are the possible triggers of blood clotting?
  4. Q) What are the implications for health of blood clotting?
  5. Q) What does a blood clot contain?
  6. Q) Why is haemophilia a health risk?

Steps: each table pick a question, discuss and choose a representative. Rep speaks, group supports, others invited to comment.

Recommended resources: Oxford text 304. Web.

LESSON TWO – Repelling an invasion – The immune system

Objectives: Understand what is meant by an immune response


  • Phagocytes / Lymphocytes: white blood cells that respond to an infection to create an immune response
  • Antibody: a chemical agent that slows or kills the spread of a pathogen inside the body
  • Antigen: surface membrane proteins that allow the immune system to recognise a pathogen
  • Endocytosis: the uptake / ingestion of a substance into a cell, by enveloping it in the membrane. (Phagocytosis is the endocytosis of solid particles, like bacteria).
  • Lysosome: a part of a cell that contains powerful digestive enzymes (white blood cells use them to destroy ingested bacteria

The immune system is designed to destroy pathogens which have successfully crossed the barrier system and invaded the body.

The immune system consists of two main kinds of white blood cells:

  • Phagocytes (ingest pathogens)
  • Lymphocytes (make antibodies)

1. Phagocytes

What does it mean when a wound is infected? 

Phagocytes are white blood cells which travel around the body, squeezing out of the walls of the capillaries to enter tissue fluid or lymph. They ingest pathogens (endocytosis), and destroy them using the enzymes in lysosomes. Phagocytes accumulate around infected wounds, resulting in a white fluid called pus.

image credit: wikihow

image credit: wikihow

Q) Where in the body can you find phagocytes?

2 Lymphocytes:

Lymphocytes produce antibodies: chemical agents that slow down the spread of pathogens in the body. Antibodies work by binding to the pathogens, either preventing their entry into host cells, or making them recognisable to phagocytes.

A typical response to an invasion of a specific pathogen:

  1.  A pathogen enters the blood stream
  2. Specialised white blood cells recognise this pathogen from its surface proteins called antigens (these are memory cells)
  3.  A large number of the lymphocytes which make the correct antibody for the pathogen are produced
  4. The pathogen is neutralised by a combination of lymphocytes, working together with phagocytes.


The first antibiotic, penicillin, was ‘first’ discovered by Sir Alexander Flemming while observing fungal growth (the actual medicine penicillin was extracted and developed by Florey and Chain).

An antibiotic is a chemical that can kill or slow down the growth of bacteria.

Antibiotics work by blocking metabolic pathways, or by disrupting protective cell walls, in prokaryotic cells only.


CHALLENGE: Explain the effects of antibiotics A, B, C. On the growth of bacteria 1, and bacteria 2. The bacterial colonies were photographed as they grew on an agar plate which had been contaminated with discs which have been soaked  image


Q) Why don’t antibiotics work on viruses?

A) Viruses are non-living in that they do not have a metabolism. Viruses work by entering a host cell, and relying on the host cell’s metabolic processes. In this way, it is difficult to target viruses without harming the host cells. None of the commonly used antibiotics are effective against viruses.


Antibiotic resistance

Antibiotic resistance means that a specific antibiotic is ineffective against a strain of bacteria. Some strains of bacteria have evolved with genes that confer resistance of antibiotics – some even have multiple resistance to different antibiotics.

Contributing factors:

  • Doctors prescribing antibiotics for infections which turn out to be viral
  • Patients failing to complete their antibiotics courses
  • Farmers adding antibiotics to livestock feed
  • Sharing of DNA between bacteria using piliabx-web-slider-chickens_0
image caption: microbionline.com

this is how bacteria do it image caption: microbionline.com






As antibiotic resistance spreads in established antibiotics, it is necessary to find new antibiotics. Fungi still represent best source of natural antibiotics. Interestingly, if an antibiotic is discontinued, it is possible for bacterial resistance to that antibiotic to wane and the antibiotic may become widely effective again.



Task: A new antibiotic is being introduced (Ticocillin). Taking the part of a representative from different interest groups, draft a piece of legislation which will allow satisfy all of your views, and allow that antibiotic to be used appropriately. Your groups should include:

  • Head of a hospital
  • Farmers union leader
  • A local politician
  • The company selling the antibiotic

Discussion points: how long should tests be carried out before this Ticocillin is allowed on the market. Who should be allowed to use it. What guidelines should they have to follow? In time, should the policy change after the antibiotic has been on the market for a while

HIV and the effect on the immune system

HIV (Human-Immunodeficiency virus) is a virus which targets a kind of white blood cell called T-Helper cells. T-helper cells help B lymphocytes make antibodies for specific antigens.

Q) Predict the effects of an HIV infection on the immune system in fighting an infection?

Discuss how awareness of HIV / AIDS is spread in your country?

Population growth Problems:

1) Revise the google link on population data.

2) Describe the changes in the bacterial population (3)

3) Explain the changes in the bacterial population (2)

4) Calculate the rate of increase for bacteria between day 1 and day 4 (2).



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