Infectious Diseases, Immunity and Vaccination
Senior 2 · Biology
Infectious Diseases, Immunity and Vaccination
Lesson 5 of 5
25 min
Measles once killed and blinded many young children in Rwanda, and polio left many children paralysed. Today both are rare here, because nearly every child is vaccinated. A vaccine gives your body the memory cells you met in the last lesson without you first having to suffer the disease. In this lesson you will classify the kinds of immunity, see how vaccines and herd immunity work, and meet the unusual case where the immune system attacks the body itself.
Immunity is the ability to resist a particular infection because antibodies or memory cells against it are already present. It is classified in two ways.
Each kind can be gained naturally or artificially (through medical action):
| Natural | Artificial | |
|---|---|---|
| Active | You catch a disease and recover, for example chickenpox or measles | You receive a vaccine |
| Passive | Antibodies pass from mother to baby across the placenta, and in breast milk (especially the first milk, colostrum) | You are injected with antibodies, for example rabies antibodies after a dog bite, antivenom after a snakebite, or tetanus antitoxin |
A vaccine contains antigens from a pathogen, in a form that cannot cause the disease. It triggers a primary immune response and leaves memory cells, so if the real pathogen ever arrives, the fast secondary response destroys it. Vaccination is the act of giving a vaccine, by injection or by mouth. Immunisation is the process of becoming immune; vaccination is the most common way to immunise someone.
| Type of vaccine | What it contains | Examples |
|---|---|---|
| Live attenuated | A living but weakened form of the pathogen | measles–rubella, BCG (TB), oral polio, yellow fever |
| Inactivated | Whole pathogens killed by heat or chemicals | injectable polio, rabies |
| Subunit | Only certain antigens of the pathogen | hepatitis B, HPV (human papillomavirus) |
| Toxoid | A bacterial toxin made harmless | tetanus, diphtheria |
Some vaccines need extra doses called boosters, which work like repeated exposures and build up more memory cells. Rwanda's national immunisation programme gives vaccines free at health centres, starting with BCG at birth. In 2011 Rwanda became one of the first countries in Africa to vaccinate all girls against HPV, which causes most cases of cervical cancer.
When most people in a community are immune, a pathogen cannot find enough people to infect, so it dies out before it reaches those who are not protected: newborn babies, people too ill to be vaccinated, and the few people in whom a vaccine did not work. This community protection is herd immunity. Measles spreads so easily that about 95% of people must be immune to stop it.
Use the chart above. The second dose of measles vaccine (MCV2) gives full protection.
While lymphocytes mature, those that react strongly to the body's own molecules are normally destroyed, so the immune system learns to tolerate "self". Sometimes this fails. In autoimmunity, the immune system wrongly treats the body's own molecules as antigens and attacks its own tissues with antibodies (autoantibodies) or T cells. Examples include rheumatoid arthritis (joints), lupus (many organs) and myasthenia gravis (the receptors that make muscles contract).
Type 1 diabetes is an autoimmune disease. The pancreas contains small groups of cells (the islets) whose beta cells make insulin, the hormone that lets body cells take in glucose and so lowers blood glucose. In type 1 diabetes, T cells destroy the beta cells. With little or no insulin, glucose builds up in the blood.
Tip
Classify immunity with two questions
Only active immunity produces memory cells, which is why only active immunity lasts.
Classify the immunity in each case: (a) Gatete recovers from chickenpox; (b) a newborn baby is protected by antibodies from its mother's blood; (c) Keza receives the HPV vaccine; (d) a man bitten by a snake is given antivenom.
Answer: (a) natural active; (b) natural passive; (c) artificial active; (d) artificial passive.
A dog bite victim is given both rabies antibodies and a rabies vaccine on the same day. Explain why each is given.
Answer: The antibodies give immediate passive protection, attacking any virus at once while the body is not yet ready. The vaccine starts active immunity, which takes one to two weeks to build but produces memory cells for long-lasting protection. The antibodies cover the gap until the vaccine's protection takes over.
Explain how herd immunity protects a baby too young to receive the measles vaccine.
Answer: If about 95% of the people around the baby are immune, an infected person is very unlikely to meet anyone who can catch the virus, so the chain of transmission breaks and the virus never reaches the baby.
Explain why a person with type 1 diabetes has high blood glucose, and why the condition is called autoimmune.
Answer: The person's own T cells have destroyed the beta cells of the pancreas, so little or no insulin is made. Without insulin, body cells cannot take in glucose well, so glucose stays in the blood. It is autoimmune because the immune system has attacked the body's own cells as if they were foreign.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
