Routes of entry and protective barriersSpec 6.7
In short
Pathogens enter the body through the gas exchange system, the digestive system, breaks in the skin and the genital and eye mucous membranes. Barriers that protect include the skin, stomach acid, and gut and skin flora, which are non-specific because they act against all pathogens in the same way.
Major routes of entry
| Route | How | Examples |
|---|---|---|
| Gas exchange system | Breathing in droplets or spores | TB, influenza, measles |
| Digestive system | Eating or drinking contaminated food or water | Cholera, Salmonella food poisoning |
| Breaks in the skin | Cuts, wounds, bites from vectors, injections, needles | Tetanus, malaria (mosquito bite), HIV and hepatitis B (needles) |
| Mucous membranes of the genital tract and eye | Direct contact with infected fluids or surfaces | HIV, gonorrhoea, chlamydia, conjunctivitis |
| Across the placenta or in breast milk | Mother to fetus or baby | HIV, rubella |
Barriers to infection
| Barrier | How it protects |
|---|---|
| Skin | A tough layer of keratinised dead cells forms a physical barrier. Sebum from sebaceous glands contains fatty acids, making the surface acidic and hostile to many microorganisms. Cuts are sealed quickly by blood clotting. |
| Stomach acid | Hydrochloric acid keeps the stomach at about pH 1 to 2, killing most microorganisms taken in with food and drink. Pepsin works at this pH. |
| Gut flora | Harmless bacteria in the gut compete with pathogens for nutrients and space, and produce substances (such as lactic acid) that inhibit pathogens. Antibiotics can reduce gut flora and allow pathogens to grow. |
| Skin flora | Resident bacteria on the skin compete with pathogens for nutrients and space and produce acids and antimicrobial substances. Normal flora can themselves cause infection if they enter the body through broken skin. |
Other barriers are mucus, which traps microorganisms, and cilia on the lining of the airways, which sweep mucus up and out of the gas exchange system. Tears and saliva contain lysozyme (see the next subtopic).
'Gut flora fight pathogens' is too vague. Say they compete for nutrients and space and produce inhibitory substances, so pathogens cannot establish themselves.
Quick check
What is the difference between a capsid and an envelope?
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The capsid is the protein coat around the viral nucleic acid. The envelope is an outer lipid layer, taken from the host cell membrane, present in some viruses.
Why do viruses only reproduce inside host cells?
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They have no ribosomes or metabolism, so they use the host's enzymes, ribosomes and ATP.
What is a provirus?
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A DNA copy of the HIV genome made by reverse transcriptase and integrated into the host cell's chromosome.
Where does Mycobacterium tuberculosis survive in the body?
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Inside macrophages in the lungs, which cannot digest it.
Name two ways that gut flora protect against infection.
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They compete with pathogens for nutrients and space, and they produce substances that inhibit pathogens.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026