6.3 – Defence Against Infectious Diseases

6.3 – Defence Against Infectious Diseases

6.3.1 – Define pathogen

An organism or virus that causes a disease or sickness. These are usually microorganisms.


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

Antibiotics slow or kill off bacterial diseases. They are found in nature, mainly in fungi or bacteria, and slow down the growth of microorganisms. They enter the bacterial cells during their growth and division.

Only some antibiotics are safe to use as drugs. Some are called broad spectrum antibiotics because they are effective on a range of microorganisms.

Antibiotics inhibit growth by disrupting the metabolism of their target pathogens, preventing the synthesis of a new cell wall during mitosis, and causing the cell to rupture. The metabolic pathways that they target are only found in bacteria, and not in eukaryotic cells.



In some cases, the antibiotics may attack the dormant spores of the bacteria instead.

On the other hand, antibiotics have no effect on viruses because they are not living organisms, and therefore have no metabolism. Instead, a virus will replicate using the metabolism of its host cell. Hence, antibiotics should not be prescribed for viral infections.

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

Although there are many different types of bacteria that safety live in or on our bodies, others act as pathogens and need to be prevented from entering. Intact skin forms a barrier against the entry of pathogens into the body. Skin is toughened on the surface by the protein keratin. Most microorganisms cannot get past the skin. For further protection, the skin also produces chemical secretions that prevent the growth of fungi and bacteria on the skin. It also has a low pH, which would hinder the colonisation of bacteria.

If a pathogen enters the body, it will because trapped in the sticky mucous and expelled by the cilia. Tears, saliva and mucous all help to wash bacteria away. Many pathogens may try to enter the lungs, but are prevented by the mucous in the trachea, bronchi and bronchioles. Some cells have cilia which move the mucous up to the epiglottis, where the pathogens can then be swallowed and killed in the acidic environment of the stomach.


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

Phagocytes are a form of leukocytes, and are part of the body’s second line of defence against disease. They ingest bacteria, viruses and dust particles and destroy them. They are able to change shape so that they can ingest microbes.

The membrane of the phagocyte changes shape to surround the microbe, then joins together to form a vesicle, or phagozome. The lysosomes in the phagocyte then fuse with it, releasing its enzymes to kill the microbes. This process is called phagoctyosis.


6.3.5 – Distinguish between antigen and antibodies

Antigen – A foreign substance that stimulates the production of antibodies. It is recognised
by the immune system, triggering this immune response.
Antibodies – Proteins, immunoglobin, that recognise and bind to specific antigens. These
have a T or Y shape made from polypeptide chains.


6.3.6 – Explain antibody production

Antibodies are produced by the lymphocytes, which are a form of leukocytes. The lymphocytes are specialised so that each one makes a specific antibody, so there is a huge range of different lymphocytes in the body.

The antibodies form on the surface of the lymphocyte membrane so that the antigen combining site points outwards. These sites will bind with specific antigens on the pathogens.

When the antibody and antigen bind, the lymphocyte becomes activated, and will begin cloning itself through mitotic division. The clone cells will also make antibodies to help fight the pathogen and defend the body.

6.3.7 – Outline the effects of HIV on the immune system

HIV stands for the Human Immunodeficiency Virus. It is a retrovirus, which means it uses RNA to make DNA. The virus contains two strands of RNA inside a protein coat and the enzyme reverse transcriptase. The virus takes advantage of the lysogenic cycle to replicate the virus within the human’s cells. It is capable of resting dormant.

When the virus enters the host cell, the RNA is translated into DNA, and then inserted in the DNA of the cell. From here, the virus in replicated and will remain with the cell even after it divides.

The outside of the virus is covered in binding proteins which attach to the CD4 receptors on helper T-cells. The virus capsule will then fuse with the membrane of the cell and the RNA is able to enter. However, the outside of the virus, which has the antigens, remains on the outside of the cell.

HIV remains dormant for a long period of time. It is not until about 10 years later that the HIV genes may become activated and the cell begins to produce the virus. As it is continuously replicated, the new viruses will infect other T-cells.

The key effects of the HIV virus are a reduction in the number of active lymphocytes and a loss of ability to produce antibodies. Since the lymphocytes are essential for defending the body against pathogens, the immune system is significantly weakened by HIV, allowing diseases to infect the body.

It is difficult to fight the virus because it targets HTC cells, which conduct immune responses. This damages the immune system because it becomes depressed and unable to recognise a pathogen. The B-cells are not properly activated so that they have limited AMI response. Also, fewer B-cells are produced which means that they have less memory cells produced. There is limited activation of KTC so that the person has limited CMI response.

Since the immune system becomes depressed, a small illness could kill the person. Over time, the body has fewer and fewer lymphocytes, resulting in fewer antibodies, and the body is vulnerable to infections.


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

AIDS is acquired immune deficiency syndrome. It is the disease that develops when the body becomes infected with the HIV virus. The immune system becomes depressed and the body is vulnerable to infection.

The negative social implications include:

  • Many consider it to be the “Gay Disease,” causing discrimination against those who are homosexual or have AIDS
  • Individuals who have AIDS may be shunned because those who do not understand it may think that they will contract it if they make contact with the person.
  • Those with limited understanding of the disease may have unreasonable fear of it.
  • Families affected by AIDS suffer grief for the victim  Victims may suffer from guilt or loneliness
  • Increased poverty and unemployment, as infected people may find it harder to work and may be refused life insurance. Housing may also refused or difficult to obtain.
  • Sexual activity in the population is reduced.
  • Children may be orphaned, and responsibility on older siblings may be increased so that they cannot continue their education
  • Young children may be infected at birth, leading to early death
  • Treating AIDS patients may lead to other diseases not being treated properly

The positive social implications include:

  • More research is done on disease
  • There is greater public awareness of safe sexual behaviour
  • Sterile needles are given to drug users, making the process safer
  • Blood is screened during blood transfusions to ensure there is no disease present
  • There is treatment available for those with HIV

AIDS is caused by the HIV virus, which can be transmitted though:

  • Vaginal, anal or oral intercourse, especially if there are cuts or tears present
  • Sharing of hypodermic needles between drug abusers.
  • The placenta, from mother to child
  • Blood transfusion
  • Cuts during childbirth or milk from breast feeding
  • Blood factors used to treat haemophilia