The number of new cases of a disease rose from 1250 in one week to 1600 in the next. Calculate the percentage change.
- Change = new − old = 1600 − 1250 = 350.
- Percentage change = 350 ÷ 1250 × 100 = 28.
Answer: An increase of 28%.
Defence against disease
Biology · C Interaction and interdependence. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
A pathogen is an organism or virus that causes disease. The skin and mucous membranes are the first barrier. If the skin is cut, a clot seals the wound.
The innate immune system acts at once and in the same way against every pathogen: phagocytes engulf pathogens and digest them. The adaptive system is slower but specific. Lymphocytes recognize one particular antigen and make antibodies against it, and memory cells remain afterwards. A second infection by the same pathogen meets a faster and much larger antibody response, which is immunity.
Vaccines use this memory. Antibiotics are different: they are drugs that kill bacteria or stop them growing.
Rules to remember
- Clotting: platelets release clotting factors → thrombin is formed → thrombin changes soluble fibrinogen into insoluble fibrin → the fibrin mesh traps blood cells.
- Antibody production: antigen activates a helper T-cell → the helper T-cell activates the matching B-cell → the B-cell divides to form plasma cells (secrete antibodies) and memory cells.
- HIV is passed on in body fluids (unprotected sex, shared needles, blood, mother to child). It destroys helper T-cells, so antibodies cannot be made and AIDS develops.
- Antibiotics block processes of prokaryotic cells only, so they do not harm human cells and do not work on viruses.
- Resistance: a few bacteria are resistant by chance; the antibiotic kills the rest; the resistant ones multiply (natural selection).
- Zoonosis: a disease that passes from other animals to humans. Herd immunity: so many people are immune that the pathogen cannot spread to those who are not.
- percentage change = (new value − old value) ÷ old value × 100.
Common mistake
Students write that bacteria become resistant because the antibiotic makes them mutate. The resistant bacteria were already there; the antibiotic only selects them.
Worked example
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