(a) Explain the following terms: (i) disease (ii) symptoms of diseases. (b)(i) List two physical and two chemical barriers that prevent pathogens from penet...

Answer Details

(a) Explanation of terms

• (i) Disease - A disease is any disorder or departure from the normal state of health of the body, in which the normal functioning of a part or the whole of the body is disturbed, usually accompanied by characteristic signs and symptoms.
• (ii) Symptoms of disease - Symptoms are the observable or felt signs and abnormal changes in the body that indicate the presence of a disease (for example fever, headache, weakness, vomiting or rashes).

(b)(i) Two physical and two chemical barriers

Physical barriers: the skin (intact epidermis) which keeps out pathogens; hairs and mucus / cilia lining the respiratory tract that trap microbes.

Chemical barriers: hydrochloric acid in the stomach which kills swallowed bacteria; lysozyme enzyme in tears, saliva and sweat which destroys bacteria (sebum on the skin is also chemical).

(b)(ii) How vaccination protects the body from infectious diseases

A vaccine contains dead, weakened (attenuated) or modified pathogens, or their harmless toxins (antigens). When introduced into the body, these antigens stimulate the white blood cells (lymphocytes) to produce specific antibodies against that pathogen without causing the disease. The body also produces memory cells. If the real pathogen later enters the body, the memory cells enable the body to produce the correct antibodies rapidly and in large amounts, destroying the pathogen before it can cause disease. This gives the body active, long-lasting immunity.

(c) Difference between an antibody and an antigen

An antigen is a foreign substance (often a protein on the surface of a pathogen) that, when it enters the body, stimulates the production of antibodies. An antibody is a protein produced by the white blood cells (lymphocytes) in response to an antigen, which combines with and neutralises or destroys that specific antigen.

(d) Causative agents

• (i) Malaria - a protozoan, Plasmodium.
• (ii) Cholera - a bacterium, Vibrio cholerae.
• (iii) AIDS - a virus, the Human Immunodeficiency Virus (HIV).