The human body has a number of barriers that prevent pathogens from entering the body:

- The skin

- Sebum

- Tears, saliva and urine

- Mucus in respiratory tracts

- Commensals

- Stomach acid and enzymes

If the body is injured, a general inflammatory response will take place:

- blood clots at the site of a cut, which seals the cut, to prevent pathogens entering the blood

- inflammation around the site of the injury takes place

- Macrophages are attracted to the site of injury, where they destroy pathogens by phagocytosis

There are four main types of immunity:

1. Active natural immunity: memory cells develop after natural exposure to antigens

2. Active artificially induced immunity: memory cells develop after vaccination

3. Passive natural immunity: antibody transfer (e.g. through placenta or breast feeding) results in short-term immunity (a few months), as no memory cells develop

4. Passive artificially induced immunity: antibodies are injected (short-term immunity)

 It is difficult to become immune to a virus, as they undergo many mutations, resulting in many different forms producing similar symptoms, but with different antigens

Antibiotics are only effective against bacterial infections

Antibiotics work in a number of ways:

- Inhibiting cell wall synthesis

- Binding to ribosomes and inhibiting protein synthesis

- Interfering with prokaryotic DNA replication and transcription

- Binding to the cell membrane to make it more permeable

- Inhibiting cell metabolism

Bacteria may become resistant to antibiotics by altering the structure of the antibiotic or by modifying the bacterial cells.

Resistance develops as a spontaneous mutation, and can spread through the population asexually and sexually