Defence against infectious disease

6.3.1 Define pathogen.

Pathogen: an organism or virus that causes a disease.

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

Antibiotics are produced by microorganisms to kill or control the growth of other microorganisms by blocking specific metabolic pathways within the cell. Since bacteria are so different to human cells, antibiotics can be taken by humans to kill bacteria without harming the human cells. Viruses on the other hand are different as they do not carry out many metabolic processes themselves. Instead they rely on a host cell (a human cell) to carry out these processes for them. Therefore viruses cannot be treated with antibiotics as it is impossible to harm the virus without harming the human cells. 


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

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

The skin forms a physical barrier that prevents pathogens from entering the body as the outer layer is very tough. In addition 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 form another type of barrier against pathogens. Mucous membranes are soft and moist areas of skin found in the trachea, nose, vagina and urethra. These membranes are not strong enough to create a physical barrier but they do have mucus which contain lysozyme enzymes that digest the phagocytes. Also, the mucus can be sticky such as in the trachea, and trap the pathogens which are then expelled up the trachea and out of the body by muscles within the trachea. 



  1. Forms a physical barrier.
  2. Sebaceous glands secret lactic acid and fatty acids. 

Mucous membranes:

  1. Mucous contains lysozyme enzymes. 
  2. Mucous can be sticky and trap pathogens.

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

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 can pass through the pores of capillaries and into these tissues.

6.3.5 Distinguish between antigens and antibodies.

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

6.3.6 Explain antibody production.

Lymphocytes are a type of leukocyte which make antibodies. Each lymphocyte makes only one specific antibody. A large amount of different lymphocytes are needed so that the body can produce different types of antibodies. The antibodies are found on the surface of the plasma membrane of these lymphocytes with the antigen-combining site projecting outwards. Pathogens have antigens on their surface which bind to the antigen-combining site of the antibodies of a specific lymphocyte. When this happens the lymphocyte becomes 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 pathogen.


  1. Each lymphocyte makes one type of antibody.
  2. Antibodies are found on the surface of the lymphocyte.
  3. Pathogen have antigens on their surface.
  4. The antigens bind to the antibodies. 
  5. Lymphocyte becomes active and makes clones of itself. 
  6. The clones make more of the specific antibody.

6.3.7 Outline the effects of HIV on the immune system.

The HIV virus (which causes AIDS) destroys a type of lymphocyte which has a vital role in antibody production. Over the years this results in a reduced amount of active lymphocytes. Therefore, less antibodies are produced which makes the body very 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.

6.3.8 Discuss the cause, transmission and social implications of AIDS.

Cause: HIV causes AIDS (acquired immunodeficiency syndrome). A syndrome is a group of symptoms that are found together. HIV destroys a type of lymphocyte which is vital for antibody production. Over the years, less active lymphocytes are produced which leads to a fall in the amount of antibodies. Pathogens that would normally be easily controlled by the body in healthy individuals can cause serious consequences and eventually lead to death for patients affected by HIV. The immune system is considerably weakened. 

Transmission: HIV is transmitted through body fluids from an infected person to an uninfected one. This can occur through vaginal and anal intercourse as well as oral sex if there are cuts or tears in the vagina, penis, mouth or intestine. It can also be transmitted by hypodermic needles that are shared by intravenous drug abusers. The small amount of blood present on these needles after their use may contain the virus and is enough to infect another person. Another way of transmission is through the placenta from mother to child, or through cuts during childbirth or in milk during breast feeding. Finally there is a risk of transmission in transfused blood or with blood products such as Factor VIII used to treat hemophiliacs. 

Social implications: Relatives and friends suffer grief. Families can also suffer from a loss of income as the person infected by HIV can lose their wage if they are unable to work and are refused life insurance. Also, HIV patients may find it hard to find partners, employment and even housing. Finally, AIDS can cause fear in a population and reduce sexual activity. 



  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.


  1. Through vaginal and anal intercourse as well as oral sex if cuts or tears are present. 
  2. Through hypodermic needles shared by drug users.
  3. Through placenta from mother to child.
  4. Through cuts during child birth or in milk during breast feeding.
  5. Through transfused blood.
  6. Through blood factors such as Factor VIII used to treat hemophiliacs. 

Social Implications: 

  1. Grief suffered by relatives and friends.
  2. Families can get poorer. 
  3. Can be hard to find a partner, employment and housing.
  4. Can reduce sexual activity in a population.