Infection and response
Biology · GCSE · The School
Four kinds of pathogen
BACTERIA are living cells that divide rapidly and often damage tissue by producing toxins. VIRUSES are not cells; they enter a host cell, use it to make copies, and burst it. PROTISTS are single-celled organisms, often carried by a vector such as a mosquito. FUNGI grow into tissue. Which one causes an illness decides what can treat it.
How the body defends itself
First the barriers: skin, mucus, cilia, stomach acid, tears. If a pathogen gets past, white blood cells act in three ways — engulfing it (phagocytosis), producing ANTIBODIES that match its surface antigens, and producing antitoxins. The antibody response is SPECIFIC: it recognises that pathogen and no other, which is why the second infection is usually milder.
Vaccination and antibiotics
A vaccine introduces a dead or inactive form so the immune system produces antibodies and MEMORY cells without the illness. Antibiotics kill bacteria; they do nothing to viruses, so taking them for a cold treats nothing and helps resistant bacteria survive. RESISTANCE arises because the few bacteria that survive a course reproduce, and their offspring inherit the resistance.
Measles is a VIRUS — antibiotics do nothing; prevention is by vaccination. Salmonella is a BACTERIUM — antibiotics work; prevention is hygiene and cooking. Rose black spot is a FUNGUS — treat with fungicide and remove affected leaves. Malaria is caused by a PROTIST spread by mosquitoes — control the vector with nets and drainage. The treatment follows the type, which is why identifying the type comes first.