Diseases and immunity

Cambridge IGCSE Co-ordinated Sciences 0654 and Combined Science 0653 · Core and Extended

Syllabus points

• Define a pathogen and a transmissible disease.
• Describe how a transmissible disease is spread, by direct contact and indirectly.
• Describe the body's mechanical and chemical barriers to pathogens.
• Explain the role of white blood cells in phagocytosis and antibody production.
• Explain how vaccination gives immunity, and what active and passive immunity are.
• Explain how hygiene, waste disposal, sewage treatment and food handling control disease.

Pathogens and transmission

A pathogen is a disease-causing organism. Bacteria, viruses, fungi and protoctists can all be pathogens.

A transmissible disease is one in which the pathogen can be passed from one host to another. That is the definition to write, and the word doing the work is pathogen. Scurvy and heart disease are diseases, but nothing passes from person to person, so they are not transmissible.

Transmission happens in two ways.

• Direct contact, through blood or other body fluids. HIV is spread this way, through unprotected sex, shared needles, and from mother to child in the womb or in breast milk.
• Indirect contact, through contaminated surfaces, contaminated food or water, animals, or droplets in the air. Cholera is spread in water contaminated with sewage, and influenza in droplets from coughs and sneezes.

Keeping pathogens out

The body stops most pathogens before they ever reach a cell, and the barriers divide neatly in two.

Mechanical barriers physically block entry:

• Skin, an unbroken layer of tough, dead, keratinised cells.
• Hairs in the nose, which trap particles in the air.
• Mucus in the airways and gut, which traps pathogens in a sticky layer.

Chemical barriers destroy pathogens:

• Stomach acid, at about pH 2, kills most bacteria swallowed with food.
• Mucus again, which does both jobs at once by trapping and by carrying antimicrobial substances.

The airway defence is a favourite question and the two halves are routinely swapped. Goblet cells make the mucus, and the mucus traps the pathogens. Cilia move the mucus, sweeping it up the airway to the throat where it is swallowed. Cilia do not make mucus, and mucus does not move cilia. The reason smoking causes so much chest infection is that smoke paralyses the cilia, so the mucus and everything trapped in it stay in the lungs.

White blood cells

If a pathogen gets past the barriers, two kinds of white blood cell deal with it, and questions frequently ask which one is which.

Phagocytes carry out phagocytosis. The cell changes shape, flows around the pathogen, engulfs it, and digests it with enzymes. A phagocyte will engulf any pathogen, so this defence is fast but not specific.

Lymphocytes make antibodies. An antibody is a protein with a shape complementary to one specific antigen on the surface of one kind of pathogen, so a lymphocyte making antibodies against measles is no help at all against cholera. Antibodies work by sticking pathogens together in clumps so phagocytes can engulf them more easily, by marking them for destruction, and by neutralising their toxins.

The specificity is what makes the immune response slow the first time and fast afterwards. On first exposure the right lymphocyte has to be found and multiplied, which takes days, and in that time you feel ill. Some of those cells remain afterwards as memory cells, so a second exposure to the same pathogen produces antibodies far more quickly and in far greater quantity, usually before any symptoms appear. That is what being immune means.

Vaccination

A vaccine contains a weakened, dead or otherwise harmless form of a pathogen, or its antigens. Because the antigens are there but the pathogen cannot cause disease, the immune system responds as though it had met the real thing.

The sequence is worth learning as five steps, because that is how the question is marked:

1. The vaccine puts the pathogen's antigens into the body.
2. Lymphocytes recognise them as foreign.
3. The lymphocytes make antibodies specific to those antigens.
4. Memory cells are produced and remain in the body.
5. If the real pathogen ever arrives, memory cells make antibodies quickly and in large amounts, so the pathogen is destroyed before it can cause symptoms.

Say antigens, not "the disease". A vaccine does not give you a mild version of the illness in order to teach the body a lesson; it presents the recognisable surface of the pathogen without the danger.

Active immunity is protection from your own antibodies, made after meeting an antigen either through infection or through a vaccine. It takes time to develop and lasts a long time, because memory cells are made.

Passive immunity is protection from antibodies made by someone else, such as those a baby receives across the placenta or in breast milk. It works immediately, which is its whole point, but it is short-lived, because no memory cells are produced and the borrowed antibodies are broken down.

If enough of a population is vaccinated, the pathogen cannot find enough susceptible people to spread through, so even those who are not vaccinated are protected. Smallpox was eliminated worldwide this way.

Controlling the spread

• Personal hygiene, particularly washing hands with soap after using the toilet and before handling food.
• Waste disposal, so that rubbish does not accumulate and attract animals that carry pathogens.
• Sewage treatment, which keeps human waste out of drinking water. This is the single most important measure against cholera.
• Safe food handling: cooking food thoroughly, refrigerating it, and keeping raw and cooked food apart.
• Access to clean drinking water.

Antibiotics kill bacteria but have no effect on viruses, because a virus is not a cell and has none of the structures antibiotics attack. Prescribing them for a cold or for influenza therefore does nothing, and overuse of them is what allows resistant strains of bacteria to be selected.

Common mistakes

• Saying a pathogen is a disease. It is the organism that causes one.
• Calling every disease transmissible.
• Saying cilia make mucus, or that goblet cells move it.
• Saying phagocytes make antibodies, or that lymphocytes engulf pathogens.
• Saying a vaccine contains antibodies. It contains antigens, and the body makes the antibodies itself.
• Saying a vaccine gives you a mild dose of the disease.
• Saying antibodies kill pathogens directly, without mentioning clumping, marking or neutralising toxins.
• Calling the immunity from breast milk active. It is passive, because the antibodies were made by the mother.
• Saying antibiotics cure influenza.
• Saying antibodies are non-specific, or that one antibody works against several pathogens.