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Immunity and disease: Class 12 Biology
Pathogens and the diseases they cause, how infections spread, body defences, the immune response with antibodies and lymphocytes, types of immunity, vaccination, allergy, HIV and AIDS, and control of disease.
Key points
A disease is a disorder that stops the body from working normally. An infectious (communicable) disease is caused by a pathogen and can pass from one person to another. A non-infectious disease, such as diabetes or heart disease, is not caused by a pathogen. Pathogens include bacteria, viruses, fungi, protozoa and parasitic worms.
Examples of infectious diseases in Pakistan:
- Tuberculosis (bacterium): spreads by droplets from coughs and affects the lungs.
- Cholera and typhoid (bacteria): spread through contaminated water and food.
- Malaria (protozoan Plasmodium): carried by female Anopheles mosquitoes.
- Dengue (virus): carried by Aedes mosquitoes.
- Hepatitis B and C (viruses): spread through blood and unsafe needles.
- Poliomyelitis (virus): spreads from faeces to mouth and damages nerves. It is prevented by vaccine.
Pathogens spread by direct contact, droplets in the air, contaminated water or food, vectors (animals such as mosquitoes and houseflies that carry the pathogen), blood and body fluids, and from mother to baby. A person who carries a pathogen without symptoms is a carrier. Good hygiene, clean water, sanitation and vector control cut the spread.
The body has barriers against infection. The skin is a tough barrier, and sebum is slightly acidic. Mucus and cilia in the airway trap and sweep out microbes. Tears and saliva contain lysozyme, an enzyme that digests bacterial walls. Stomach acid kills many microbes in food. Useful bacteria on the skin and in the gut compete with pathogens. These are the non-specific first line of defence.
If a pathogen gets in, white blood cells respond. Phagocytes (neutrophils and macrophages) engulf microbes by phagocytosis and digest them with enzymes. Inflammation brings more blood and phagocytes to the site, causing redness, heat and swelling. Fever raises the temperature, which slows the growth of some pathogens. Platelets and clotting seal wounds and stop entry of microbes.
The specific immune response is carried out by lymphocytes. An antigen is a molecule, usually a protein on the surface of a pathogen, that the body treats as foreign. B lymphocytes make antibodies, which are Y shaped proteins whose tips fit a particular antigen. Antibodies stick pathogens together, neutralise toxins and mark pathogens for phagocytes. T lymphocytes include helper T cells, which coordinate the response, and killer T cells, which destroy infected body cells.
After the first meeting with an antigen, some B and T cells become memory cells which stay for years. On a second exposure the memory cells respond faster and make more antibodies, so the person usually does not become ill. This is immunological memory. It is the basis of long term immunity.
Types of immunity. Active immunity is when the body makes its own antibodies. Natural active immunity follows an infection. Artificial active immunity follows vaccination. Passive immunity is when ready made antibodies are given. Natural passive immunity is by antibodies passing from mother to baby through the placenta and in colostrum. Artificial passive immunity is by injection of antibodies, such as antivenom for snake bite. Passive immunity acts at once but lasts only a short time because no memory cells form.
A vaccine contains a weakened or killed pathogen, a harmless part of it, or a modified toxin. It causes the body to form antibodies and memory cells without causing disease. Examples are the BCG vaccine against tuberculosis, the polio drops and the measles vaccine. If most of a population is vaccinated, the pathogen cannot spread easily, and unvaccinated people gain some protection. This is herd immunity. Smallpox was removed from the world by vaccination.
Allergy is an over-reaction of the immune system to a harmless substance such as pollen, dust or some foods. Histamine is released and causes sneezing, itching and swelling. Autoimmune diseases occur when the immune system attacks the body's own cells, as in type 1 diabetes. In transplant rejection the antigens of the donor organ are seen as foreign, so the patient takes drugs to suppress the immune response.
HIV (human immunodeficiency virus) infects and destroys helper T cells, so the immune system gets weaker. After years the person develops AIDS (acquired immunodeficiency syndrome) and is open to other infections and some cancers. HIV spreads by unprotected sex, infected blood or needles, and from mother to child. It does not spread by touching, sharing food or mosquito bites. Antiviral drugs control but do not cure it. Antibiotics act on bacteria and not on viruses.
Study tip: Make a table of common diseases with pathogen, mode of spread and prevention. Learn the stages of the immune response in order (antigen, B cell, antibody, memory cell), and always state the type of immunity with a clear example.
Definitions and flashcards
Say the meaning first, then open the card to check.
Pathogen
A microorganism that causes disease.
Antigen
A substance, usually a protein, that causes the immune system to make antibodies.
Antibody
A protein made by B lymphocytes that binds to a specific antigen.
Phagocytosis
The engulfing and digestion of microbes by white blood cells.
Vaccine
A preparation that causes immunity without causing the disease.
Memory cell
A long lived lymphocyte that responds quickly when the same antigen is met again.
Vector
An organism that carries a pathogen from one host to another.
Active immunity
Immunity in which the body makes its own antibodies.
Herd immunity
Protection of a community when enough people are immune.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Differentiate between an antigen and an antibody. (2 marks)
An antigen is a foreign molecule, usually on a pathogen, that triggers the immune response. An antibody is a protein made by B lymphocytes that binds specifically to that antigen.
What is a vector? Name a disease spread by one. (2 marks)
A vector is an organism that carries a pathogen from an infected host to another host. Malaria is spread by female Anopheles mosquitoes, and dengue by Aedes mosquitoes.
Describe the non-specific defences of the human body. (3 marks)
The skin forms a barrier. Mucus and cilia in the airway trap microbes and sweep them out. Lysozyme in tears and saliva digests bacterial walls and stomach acid kills microbes in food. If microbes enter, phagocytes engulf them and inflammation brings more white cells to the area. These work against all pathogens in the same way.
Explain why a second infection with the same pathogen is often less severe. (3 marks)
During the first infection some B and T lymphocytes become memory cells that remain in the body. If the same antigen enters again, memory cells divide rapidly and produce large amounts of antibody much faster. The pathogen is destroyed before it can cause symptoms.
Differentiate between active and passive immunity with examples. (3 marks)
In active immunity the person makes antibodies after an infection or a vaccine. It starts slowly but lasts a long time because memory cells form. In passive immunity ready made antibodies are given, as from mother to baby by colostrum, or by antivenom. It acts at once but is short lived and has no memory cells.
Explain how HIV leads to AIDS. (5 marks)
HIV enters the body through blood or body fluids and infects helper T lymphocytes. The virus copies itself inside them and destroys them. Helper T cells are needed to activate B cells and killer T cells, so as their number falls, the immune system cannot respond properly. After several years the person has AIDS and suffers repeated infections, such as tuberculosis and pneumonia, and some cancers that a healthy person would resist. Drugs can slow the virus but do not cure the disease, so prevention by safe blood, sterile needles and education is important.
Common mistakes
- Saying antibiotics kill viruses. They act only on bacteria.
- Confusing a vaccine (gives active immunity) with an antiserum such as antivenom (gives passive immunity).
- Writing that antibodies attack the pathogens like killers. They bind to antigens and help other cells destroy them.
- Saying that HIV and AIDS are the same thing. HIV is the virus, AIDS is the later condition.
- Mixing up the vector and the pathogen in malaria. The mosquito is the vector and Plasmodium is the pathogen.