6.3: Defence against infectious diseases

Question 1

Define Pathogen

Answer 1

An organism or virus that causes a disease.

Question 2

Explain why antibiotics are effective against bacteria but not against viruses.

Answer 2

1. Antibiotics block specific metabolic pathways in bacteria. Bacteria are very different to human cells so human cells are not affected.
2. Viruses require a host cell (human cell) to carry metabolic processes for them and so antibiotics cannot be used to treat viruses. Harming the virus would harm the human cells.

Question 3

Outline the role of skin and mucous membranes in defence against pathogens:

Answer 3

Skin:

• Forms a physical barrier that prevents pathogens from entering the body as the outer layer is very tough.
• The skin contains sebaceous glands which secret lactic acid and fatty acids which creates an acidic environment on the surface of the skin preventing the growth of pathogens.

Mucous membranes:

• Are soft and most areas of skin found in the trachea, nose, vagina and urethra. Mucus contains lysosome enzymes that digest the phagocytes.
• Mucus can be sticky and trap the pathogens which are then expelled out of the body.

Question 4

Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues:

Answer 4

• Phagocytes are found in the blood and ingest pathogens. They do so by recognising pathogens and engulfing them by endocytosis.
• Enzymes within the phagocytes called lysosomes then digest the pathogens. Phagocytes can ingest pathogens in the blood but also within body tissue as they pass through the pores of capillaries and into these tissues.

Question 5

Distinguish between antigens and antibodies:

Answer 5

1. Antibodies are proteins that defend the body against pathogens by binding to antigens on the surface of these pathogens and stimulating their destruction.
2. Antigens are foreign substances which stimulate the production of antibodies. Antibodies usually only bind to one specific antigen.

Question 6

Explain antibody production:

Answer 6

Lymphocytes are a type of leukocytes which make antibodies. Each lymphocyte (B-cells) makes only one specific antibody. Large amounts of lymphocytes are needed so that there are many different antibodies. The antibodies are found on the surface of the lymphocytes (helper T lymphocytes) with the antigen-combining site projecting outwards. Pathogens have antigens on their surface which bind to the antibodies. When this happens, the lymphocytes become active and starts to make clones of itself by dividing by mitosis. These clones then start to make more of this specific antibody needed to defend the body against the specific pathogen.

Question 7

Outline the effects of HIV on the immune system:

Answer 7

The HIV virus (which causes AIDS) destroys a type of lymphocyte (B-cells) which has a vital role in the production of antibodies. Over the years this results in a reduced amount of active lymphocytes. Therefore, less antibodies are produced which makes the body ver vulnerable to pathogens. A pathogen that could easily be controlled by the body in a healthy individual can cause serious consequences and eventually lead to death for patients affected by HIV.

Question 8

Discuss the cause, transmission and social implications of AIDS:

Answer 8

Cause:

1. HIV causes AIDS
2. HIV destroys a type of lymphocyte vital for antibody production.
3. Overtime there are less active lymphocytes.
4. The body becomes very vulnerable to pathogens.

Transmission:

• Through vaginal and anal intercourse as well as oral sex if cuts or tears are present.
• Through shared needles by drug users.
• Through placenta from mother to child.
• Through cuts during child birth or in milk during breast feeding.
• Through transfused blood.

Social Implications:

• Grief suffered by relatives and friends.
• Families can get poorer.
• Can be hard to find a partner, employment and housing.
• Can reduce sexual activity.