Pathogens and the four body defences
Pathogen = a disease-causing organism (bacteria, viruses, fungi, protoctists). A transmissible disease is one passed from one host to another. Four defences stand in the way: skin, a physical barrier; mucus from goblet cells traps inhaled pathogens while cilia sweep it up to the throat to be swallowed; stomach acid (hydrochloric acid, pH ~2) destroys pathogens in food and drink; white blood cells — phagocytes engulf, lymphocytes make antibodies.
Antigen, antibody and active immunity
Antigen = a molecule (usually a protein) on the surface of a cell that stimulates production of an antibody. Antibody = a protein produced by lymphocytes in response to a specific antigen, shaped to bind that antigen and no other. Active immunity = the body producing its own antibodies after exposure to antigens, by infection or by vaccination; it lasts because memory lymphocytes persist. A vaccine supplies the antigens, so immunity forms without the disease.
(Extended) Active vs passive immunity
Active immunity: the body makes its own antibodies, so protection builds slowly over days to weeks — but memory lymphocytes persist and it is long-lasting. Sources: natural infection and vaccination. Passive immunity: ready-made antibodies arrive from elsewhere (mother via placenta or breast milk, an anti-venom or anti-tetanus injection), so protection is immediate but short-term — the antibodies are broken down and no memory lymphocytes form.
Drawn from real examiner reports.
Antibiotics are not antibodies
Antibiotics are chemical drugs that kill or inhibit bacteria, made by microorganisms such as Penicillium or synthesised. Antibodies are proteins made by lymphocytes against one specific antigen — part of your own immune response, not a medicine. Candidates substitute one word for the other right through an answer and lose every mark that depends on it.
Antibiotics and antibodies repeatedly confused: s22 Paper 42 Q4b iv
Antibiotics do nothing to viruses
Antibiotics interfere with bacterial processes such as cell wall and protein synthesis. A virus replicates inside a host cell using the host's own machinery, so there is nothing for the antibiotic to attack. Never write that antibiotics treat influenza or HIV — antiviral drugs target viral processes instead.
Antibiotics stated to treat viral infections: s23 Paper 11 Q30; w22 Papers 11/21 Q27/Q29
HIV is a virus, not a bacterium
HIV (human immunodeficiency virus) is a virus. It infects and destroys lymphocytes, so fewer antibodies can be produced and the person becomes vulnerable to opportunistic infections — this is AIDS. Naming HIV as a bacterium usually drags a second error along with it: the claim that antibiotics would treat it.
HIV identified as a bacterium: s22 Paper 31 Q2b i
Phagocyte and lymphocyte swapped
Phagocytes carry out phagocytosis — they engulf a pathogen in a vacuole and digest it with enzymes. Lymphocytes produce antibodies specific to one antigen. Neither does the other's job. On the standard two-mark white blood cell question, giving the wrong cell the wrong function scores zero for that mark.
Phagocytes labelled as antibody producers, lymphocytes as performing phagocytosis: w22 Paper 22 Q18; s22 Paper 23 Q18
Vaccines carry antigens, not antibodies
A vaccine supplies antigens — dead or attenuated pathogens, or parts of them. The body then makes its own antibodies, which is exactly why vaccination gives active immunity. Writing that the vaccine contains or injects antibodies describes passive immunity instead, and loses the first marking point every time.
The antibiotic does not cause the mutation
Resistance does not appear because the antibiotic triggers a mutation, and bacteria do not mutate on purpose. Mutations are random and already present in the population before exposure. The antibiotic acts as a selection pressure: non-resistant bacteria die, the resistant ones survive, reproduce and pass the allele on.
Mutation described as triggered by the antibiotic: w22 Paper 41 Q4b i
Cilia sweep mucus, not bacteria
Goblet cells make the mucus, the mucus traps the pathogens, and the cilia beat to move that mucus up towards the throat, where it is swallowed. The mark scheme credits "cilia sweep mucus containing trapped pathogens". Saying cilia sweep the bacteria away skips the layer that actually carries them.
Pitch Core and Extended depth
A Core answer may stop at antigens stimulating lymphocytes to produce antibodies. An Extended answer must go further and add memory lymphocytes plus the faster, larger secondary response — leaving those out is the standard Extended mark loss.
Name the control measure exactly
For controlling the spread of a disease, name the measure: vaccination programmes, improved water supply and sanitation, quarantine of infected individuals, or vector control such as bed nets and insecticide. "Better hygiene" on its own is too vague to be credited.
The five-step vaccination answer
Antigens in without causing disease → lymphocytes stimulated → antibodies produced → memory lymphocytes formed → rapid, large secondary response on re-exposure. A 4-mark question wants four of those five steps in that vocabulary, not "the body builds up immunity".
Give both white blood cell roles
When a question asks how white blood cells defend the body, there are almost always two marks on offer: phagocytes engulf and digest pathogens by phagocytosis, and lymphocytes produce antibodies. Answering with only one of the pair leaves a mark behind.
Pathogen: a disease-causing organism. Examples: bacteria (Mycobacterium tuberculosis — TB), viruses (HIV — Human Immunodeficiency Virus), fungi (Tinea — athlete's foot), protoctists (Plasmodium — malaria).
Transmissible disease: a disease caused by a pathogen that can be passed from one host to another. Routes of transmission include direct contact, airborne droplets, contaminated water or food, vectors (e.g. mosquitoes), and sexual contact.
Antigen: a molecule (usually a protein) on the surface of a cell that can stimulate the production of an antibody. Every pathogen carries unique antigens.
Antibody: a protein produced by lymphocytes in response to a specific antigen. The antibody has a complementary shape that binds to one antigen, marking the pathogen for destruction or neutralising it directly.
Active immunity: the body produces its own antibodies following exposure to antigens — from infection or vaccination. Active immunity is long-lasting because memory lymphocytes persist.
Define (a) antigen and (b) antibody.
(a) State what is meant by a pathogen. [1]
(b) Name THREE ways in which the body prevents pathogens from entering or destroying them before they cause disease. [3]
