Defence against disease — IB Diploma Biology SL
IB Biology C3.2: pathogens, skin and clotting, innate and adaptive immunity, HIV, antibiotics, zoonoses, vaccines, herd immunity and COVID-19 data.
Revision notes
8 short notes, in the order of the specification. Each one in short:
A pathogen is a disease-causing organism, and pathogens are the cause of infectious diseases. The term is typically reserved for viruses, bacteria, fungi and protists, although a broad range of organisms can infect humans. Archaea are not known to cause any diseases in humans. Careful observation of epidemics led to major progress in controlling infectious disease.
Skin and mucous membranes are the primary defence against pathogens. The skin acts as both a physical and a chemical barrier, and mucous membranes trap pathogens in sticky mucus. Cuts are sealed by blood clotting: platelets release clotting factors that start a cascade, ending with thrombin rapidly converting fibrinogen into fibrin, which traps erythrocytes to form a clot.
The innate immune system responds to broad categories of pathogen and does not change during life, whereas the adaptive immune system responds specifically to particular pathogens and builds up a memory of them. Phagocytes, part of the innate system, reach infection sites by amoeboid movement, engulf pathogens by endocytosis and digest them with lysosome enzymes.
Antigens are recognition molecules, usually glycoproteins or other proteins on pathogen surfaces, that trigger antibody production. Lymphocytes in the blood and lymph nodes cooperate to make antibodies. A specific B-cell is activated by its antigen plus contact with a helper T-cell activated by the same antigen, then divides by mitosis into a clone of antibody-secreting plasma cells.
Immunity is the ability to eliminate an infectious disease from the body, due to the long-term survival of memory cells that can make the specific antibodies needed. Vaccines contain antigens, or DNA or RNA coding for antigens, and stimulate immunity to a specific pathogen without causing the disease. If enough of a population is immune, herd immunity greatly impedes transmission.
HIV (human immunodeficiency virus) is transmitted in body fluids such as blood, semen, vaginal fluids and breast milk. It infects and kills only certain types of lymphocyte, the helper T-cells. Losing these limits the ability to activate B-cells and produce antibodies, so the person cannot fight opportunistic infections: this is AIDS.
Antibiotics are chemicals that block processes occurring in bacteria but not in eukaryotic cells, so they kill or inhibit bacteria without harming human cells. They fail to control viral infections because viruses have no metabolism of their own. Resistance to several antibiotics has evolved in pathogenic bacteria by natural selection, so careful use is needed to slow multiresistant bacteria.
Zoonoses are infectious diseases that can transfer from other species to humans. They are common and spread in varied ways: tuberculosis from cattle, rabies through bites from infected mammals, Japanese encephalitis through mosquitoes, and COVID-19, which recently transferred from another species. Evaluating COVID-19 data involves calculating percentage change and percentage difference.
Exam questions
8 exam-style questions (27 marks), each with its mark scheme.
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23 cards: flip them, mark what you knew, and practise the rest.
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The whole of organisms (interaction and interdependence) on one page, so you can see where this subtopic fits.
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Quick check questions
Which four groups of organism is the term pathogen usually reserved for?
Show answer
Viruses, bacteria, fungi and protists.
Which enzyme converts fibrinogen into fibrin?
Show answer
Thrombin.
What two things are needed to activate a B-lymphocyte?
Show answer
Direct interaction with its specific antigen, and contact with a helper T-cell activated by the same antigen.
Why do antibiotics not work against viruses?
Show answer
Viruses have no metabolism of their own, so there are no bacterial processes to block; they use the host cell's processes.
Name three zoonoses given in the IB guide besides COVID-19.
Show answer
Tuberculosis, rabies and Japanese encephalitis.
Frequently asked questions
How do vaccines work?
Vaccines contain antigens, or DNA or RNA coding for antigens, from a specific pathogen. They trigger a primary immune response in which B-cells are activated and memory cells are produced, without causing the disease. If the real pathogen infects later, memory cells respond quickly, producing antibodies before the person becomes ill.
Why can't antibiotics kill viruses?
Antibiotics work by blocking processes found in bacteria but not in eukaryotic cells, such as cell wall synthesis or protein synthesis by 70S ribosomes. Viruses have no metabolism of their own, so there is nothing for antibiotics to block. They replicate using the host cell's processes, which antibiotics do not target.
What is the difference between innate and adaptive immunity?
The innate immune system responds to broad categories of pathogen and does not change during an organism's life; phagocytes are part of it. The adaptive immune system, based on lymphocytes, responds specifically to particular pathogens and builds up a memory of pathogens encountered, so later responses are faster and more effective.
How does HIV lead to AIDS?
HIV infects and kills helper T-cells, a type of lymphocyte. As their numbers fall, fewer B-cells can be activated, so the body produces fewer antibodies. The person can no longer fight opportunistic infections such as tuberculosis and pneumonia. This state of severe immune deficiency is called AIDS.
What is herd immunity?
Herd immunity is the protection of a whole population when a sufficient percentage of its members are immune to a disease. Infected people rarely meet susceptible people, so transmission is greatly impeded and epidemics are prevented. It protects those who cannot be vaccinated, such as newborn babies, showing that members of a population are interdependent.
Written and checked against the IB Biology SL specification · Updated October 2026