C3.2 Defence against diseaseIB Biology SL: Revision notes

A pathogen is a disease-causing organism, usually a virus, bacterium, fungus or protist. Archaea are not known to cause any human disease.

The skin is a physical barrier (tough outer layer of dead cells) and a chemical barrier (acidic secretions, antimicrobial chemicals). Mucous membranes lining the airways, gut and genital tract trap pathogens in sticky mucus.

Blood clotting seals cuts: platelets release clotting factors, triggering a cascade that produces thrombin. Thrombin rapidly converts soluble fibrinogen to insoluble fibrin, which forms a mesh that traps erythrocytes to make a clot.

NOS: careful observation drove progress — Semmelweis linked childbed fever in Vienna to doctors' unwashed hands, and Snow linked cholera in London to a contaminated water pump.

The innate immune system responds to broad categories of pathogen and does not change during an organism's life. The adaptive system responds specifically to particular pathogens and builds up a memory, so responses become more effective.

Phagocytes (innate) move by amoeboid movement out of the blood to sites of infection, recognise pathogens, engulf them by endocytosis, and digest them with enzymes from lysosomes.

Lymphocytes circulate in the blood and are also held in lymph nodes. Each person has a huge number of B-lymphocytes, each able to make one specific antibody.

Antigens are recognition molecules, mostly glycoproteins or other proteins on the surface of pathogens, that trigger antibody production. Antigens on erythrocytes can trigger antibodies if blood of a different group is transfused.

Activation: a B-cell is activated only when it binds its specific antigen and contacts a helper T-cell activated by the same antigen. Activated B-cells divide by mitosis to form a clone of plasma cells that all secrete the same antibody — needed because only a few B-cells match any one antigen.

Immunity is the ability to eliminate an infectious disease; it is due to long-lived memory cells that can rapidly make the specific antibodies.

HIV is transmitted in body fluids: unprotected sex, shared hypodermic needles, transfusion of infected blood, and from mother to baby (placenta, birth, breast milk).

HIV infects and kills helper T-lymphocytes only. Without helper T-cells, B-cells cannot be activated, so antibody production falls. The person develops AIDS and cannot fight opportunistic infections that a healthy immune system would control.

Antibiotics block processes in bacteria that do not occur in eukaryotic cells (e.g. bacterial cell wall synthesis, 70S ribosomes). They do not work on viruses, which have no metabolism of their own and reproduce using the host cell's machinery.

Resistance evolves by natural selection: a mutation (or a gene gained from another bacterium) lets some bacteria survive the antibiotic; they reproduce and resistance spreads. Some strains are now multiresistant. Careful use — no antibiotics for viral infections, completing courses, limiting use in farming — slows this.

NOS: new techniques open new research — searching chemical libraries is yielding new antibiotics.

Zoonoses are infectious diseases that transfer from other species to humans, by varied routes: tuberculosis (e.g. from cattle via milk or air), rabies (bites from infected mammals), Japanese encephalitis (mosquitoes, from pigs and birds) and COVID-19, which recently transferred from another species with profound consequences.

Vaccines contain antigens, or DNA/RNA coding for antigens, and stimulate immunity (memory cells) to a specific pathogen without causing the disease.

Herd immunity: if a high enough percentage of a population is immune, transmission is greatly impeded, protecting those who cannot be vaccinated. Members of a population are interdependent.

NOS: research is published for others to evaluate; the media often report it before evaluation is complete. Vaccines are rigorously tested, with minimal but not nil risk of side effects.

When evaluating data such as COVID-19 case rates:

• Percentage change = (new − original) ÷ original × 100. Negative values show a decrease.
• Percentage difference compares two values against a reference value in the same way, e.g. how much lower a rate is in vaccinated people than in unvaccinated people.

Always compare rates (cases per number of people) rather than raw numbers when group sizes differ, and consider sample size, controls and randomisation.