Defense Against Infectious Diseases

IB Topic 6.3

Contagion

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?

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

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

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

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)

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

Animation

• Phagocytosis

• http://www.pennmedicine.org/health_info/animationplayer/

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

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.

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.

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

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

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)

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.

Animation

• Immune response

• http://www.pennmedicine.org/health_info/animationplayer/

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.

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.

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.

Practice

Which of the following represents the correct sequence of events when the body is responding to a bacterial infection?

I. Antigen presentation by macrophagesII. Activation of B-cellsIII. Activation of helper T-cells

A. I, II, IIIB. I, III, IIC. III, II, ID. II, III, I

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

Turn and Talk

• Explain why antibiotics are used to treat bacterial but not viral diseases.

Turn and Talk

• Explain how the skin and mucous membranes prevent entry of pathogens into the body.