1. Defense Against Infectious Diseases
IB Topic 6.3

2. Contagion

3. Pathogens cause disease
Any living organism or virus that is capable of causing a disease is called a pathogen
Examples:
Viruses, bacteria, fungi worms
Exposure to the vast majority of pathogens does not cause disease
Why?

4. Antibiotics
Recall:
Our cells are eukaryotic cells and bacteria are prokaryotic
Many differences (like …) including biochemical reactions/pathways
Such as protein synthesis
Presence of a cell wall
Antibiotics are chemicals that take advantage of the differences
Antibiotics can: block protein synthesis; block ability to grow and divide

5. Do antibiotics work on viruses?NO
Viruses make use of our own body cells’ metabolism to create new viruses
Any chemical that could inhibit a virus could damage our own cells
Antibiotics damage/kill prokaryotic cells

6. How can we keep pathogens from entering our body?
Obviously, stay away from the source
Reason why people are quarantined
Skin
Epidermis is the top layer; dermis is the bottom layer
Epidermis is not truly alive – dead skin cells make for good barrier
Important to cover and clean cuts

7. How can we keep pathogens from entering our body?
Stomach acid
Some pathogens enter our system through food and water
Acidic environment helps kill most pathogens
Mucus
Pathogens enter through the air we breath
Nasal passages and mouth are covered with a mucus membrane (also found in the urethra; vagina)
Secrete sticky mucus; trap incoming pathogens
Also secrete lysozyme (an enzyme that can chemically damage many pathogens)

8. What happens when pathogens do get in?
Phagocytic leukocytes (white blood cells)
Many different types & roles
Macrophage
Large WBC
Change cellular shape to surround an invader and take it through phagocytosis
Recognizes whether the cell is natural or an invader
Based on protein molecules on the surface of all cells/viruses
Phagocytosis:
Organelle lysosome chemically digests invader
Non-specific b/c the identity of the pathogen has not been determined

9. Animation Phagocytosis

10. Antibodies are produced in response to a specific pathogen
Antibodies are protein molecules
For example, if you had a measles infection, your body would produce one type of antibody
Ditto for when you have the flu
Each type of antibody is different because each type has been produced in response to a different pathogen

11. Antigens
Antigens are molecules that our immune system considers to be “not-self”
Foreign proteins
Most pathogens have several different antigens on their surface and may trigger many different types of antibody.

12. Antibody recognition of antigen
Even though each type of antibody is different and specific for just one antigen, antibodies as a group have similarities:
Each antibody is a Y shaped protein
At the end of each Y is a binding site
The binding site is where the antibody attaches itself to an antigen.

14. How does your immune system respond?
The leukocytes that produce antibodies are called B lymphocytes (B cells)
Each of us has many different types of B lymphocytes
B cells are activated by T cells

15. How does your immune system respond?
1. A specific antigen is identified (e.g. cold virus)
2. A specific B lymphocyte is identified that can produce an antibody which will bind to the antigen (proteins on the cold virus)
3. The B lymphocyte and several identical B lymphocytes clone themselves (mitosis) to rapidly increase the number of the same type of B lymphocytes

16. How does your immune system respond?
4. The newly formed “army” begins antibody production
5. Newly released antibodies circulate in the bloodstream and eventually find their antigen match (the proteins of the pathogen)

17. How does your immune system respond?
6.Using various mechanisms, the antibodies eliminate the pathogen
7. Some of the cloned antibody-producing lymphocytes remain in the bloodstream and give immunity from a second infection by the same pathogen
They are called memory cells.

18. Animation Immune response

19. Practice What is a pathogen? A. A virus that causes a disease.
B. Any organism or virus that causes a disease.
C. A disease caused by bacteria or viruses.
D. Any organism transmitted from humans to humans.

20. Practice How do phagocytic leucocytes help to protect against disease?
A. They secrete bacterial toxins by exocytosis.
B. They ingest pathogens by endocytosis.
C. They produce antigens to destroy pathogens.
D. They produce antibodies to destroy pathogens.

21. Practice
Why are there many different types of lymphocytes in the body?
A. Each type can recognize one specific antibody and produces a specific antigen against it.
B. Each type can recognize one specific antigen and produces a specific antibody against it.
C. Each type can recognize one antigen and engulf it by phagocytosis.
D. Each type can recognize one antibody and engulf it by phagocytosis.

22. Practice
Which of the following represents the correct sequence of events
when the body is responding to a bacterial infection?
I. Antigen presentation by macrophages
II. Activation of B-cells
Activation of helper T-cells
A. I, II, III
B. I, III, II
C. III, II, I
D. II, III, I

23. Practice Which of the following best describes antibodies?
A. Made by phagocytes and specific to one antigen
B. Made by lymphocytes and specific to one antigen
C. Made by leucocytes and non-specific
D. Made by phagocytes and non-specific

24. Turn and Talk
Explain why antibiotics are used to treat bacterial but not viral diseases.

25. Turn and Talk
Explain how the skin and mucous membranes prevent entry of pathogens into the body.