What you need to know
- 6.7(i)Major routes pathogens may take when entering the body.
- 6.7(ii)Barriers: skin, stomach acid, gut and skin floraThe harmless microorganisms that normally live on the skin. They compete with pathogens..
Inflammation, phagocytosis, antibodies and lymphocytes are 6B. TB droplets and HIV in blood are examples of routes, not full 6.6 case studies.
By the end, you should be able to…
- List inhalation, ingestion and broken skin as major routes.
- Explain skin, stomach acid, gut floraThe harmless microorganisms that normally live in the gut. They compete with pathogens. and skin flora as barriers.
- Keep this page free of antibodyA Y-shaped protein made by plasma cells that binds specifically to one antigen. and T-cell stories.
Build your understanding
Three ways in
Pathogens may enter through the gas-exchange system as inhaled droplets or particles that reach airways and alveoli. They may enter through the digestive system in food or water. They may enter through the skin if it is cut, punctured or pierced by a vector or needle. Unbroken skin is a barrier, not a route.
Name the surface that is crossed. “They get into the blood” skips the route. TB typically uses the airway; many gut pathogens use ingestion; HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS. is not inhaled like TB — its body-fluid routes fit punctures and damaged mucosa, which 6A 8 will use without turning this lesson into a clinical guide.
Skin and acid
Unbroken skin is a physical wall of keratinised dead cells. Sebum and dryness add to a hostile surface. A cut bypasses keratin and lets surface microbes reach living tissue.
Stomach hydrochloric acid creates a low pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline. that denatures proteins and kills many ingested microbes. Some pathogens survive it; the barrier is still a specified reason many do not.
Flora as occupants
skin floraThe harmless microorganisms that normally live on the skin. They compete with pathogens. and gut floraThe harmless microorganisms that normally live in the gut. They compete with pathogens. are resident microbial communityAll the populations of different species living in one habitat at the same time.. They occupy space, use nutrients and attachment sites, and can make the surface harder for an invader to colonise. That is competitive exclusionWhen two species compete for the same limited resources, the better competitor eventually removes the other from the habitat. at a barrier, the same logic as 5B competition, not a specific immune response.
antibodyA Y-shaped protein made by plasma cells that binds specifically to one antigen., phagocytosisEngulfing a pathogen or particle into a vesicle by endocytosis. and inflammationA local response to infection or damage: histamine makes blood vessels dilate and become leakier, causing heat, redness and swelling and bringing in phagocytes. wait for 6B. If a question says “barriers”, stay with skin, acid and flora. The 3C microbiota extension is enrichment about communities, not extra 6.7 names.
NOT: resident flora are not the infection. They occupy the surface and compete with many newcomers.
- Name the surface that is crossed, not only the blood
- HIV is not an inhaled droplet infection like TB
- Antibodies and phagocytosis are the next topic, not these barriers
- Flora are competitive exclusion, the same idea as in ecology
- A low pH kills many ingested microbes, not every species
Two traps this diagram blocks
- Do not list antibodies as a 6.7 barrier.
- Do not call intact skin a route of entry.
From the diagram alone
Question
Describe how stomach acid and gut flora reduce infection by swallowed pathogens. [3]
Show a model answer
Stomach acid has a low pH that denatures microbial proteins and kills many of the cells. Gut flora occupy space and nutrients, so a newcomer is less able to attach and grow. Flora are competitors, not antibodies.
| Route | Barrier that meets it | How the barrier fails |
|---|---|---|
| Inhalation | Not a 6.7 named chemical barrier at the alveolus | Droplets reach gas-exchange surface |
| Ingestion | Stomach acid, then gut flora | Acid survived; flora displaced |
| Intact skin | Keratin and skin flora | Not a route until broken |
| Cut or puncture | Those barriers already bypassed | Direct access to living tissue |
Make your reasoning visible
List the four named barriers
Skin, stomach acid, gut floraThe harmless microorganisms that normally live in the gut. They compete with pathogens., skin floraThe harmless microorganisms that normally live on the skin. They compete with pathogens..
Flora compete; they do not “eat antibodies”
Keep 6B language off this page.
Avoid this misconception
The claim: Skin flora cause infection, so they cannot be a barrier.
The correction: Resident flora occupy the surface and compete with many incoming pathogens. Infection is more likely when the skin is broken or the community is disrupted, which is a different claim.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
List three major routes of pathogen entry.
Show suggested answer
Gas-exchange (inhalation), digestive system (ingestion), and broken or punctured skin (including vectors).
Practice question
Retrieval and explanation · suggested answer
How does unbroken skin act as a barrier?
Show suggested answer
Keratinised outer cells form a physical wall; skin flora and surface conditions make colonisation harder.
Practice question
Retrieval and explanation · suggested answer
How does stomach acid protect?
Show suggested answer
Hydrochloric acid produces a low pH that kills many ingested microbes by damaging proteins.
Practice question
Retrieval and explanation · suggested answer
How do gut flora protect?
Show suggested answer
They compete with pathogens for nutrients and attachment sites.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
An inhaled bacterium reaches an alveolus. Which 6.7 barrier did it not have to face, and which later topic deals with macrophages that then meet it?
Show a hint
Acid is in the gut.
Explore the reasoning
It did not face stomach acid or gut flora. Skin was not the route. Macrophages and inflammation are 6B (and TB’s survival inside macrophages is 6A 7), not extra 6.7 barriers.
Recall this lesson
Lesson recall questions
Question 1 of 5: State how stomach acid protects the body from infection.
Entry routes and barriers
Practice question
Retrieval and explanation · suggested answer
State how stomach acid protects the body from infection.
Lesson: 6A 6: Invading the Body
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Flora as ecology on a person
6A · Invading the BodyThe 3C microbiota pages already treated gut microbes as a community, not as a decoration. 6.7 uses that community as a barrier: occupation and competition. It is still…Specification 6.7(ii)Read & use the scienceClose article
Why this matters
The 3C microbiota pages already treated gut microbes as a community, not as a decoration. 6.7 uses that community as a barrier: occupation and competition. It is still not a licence to sample your own flora or to buy a probiotic as a specification answer. Different body sites offer different moisture, acidity and nutrients, so resident communities vary. Their protective effect comes partly from occupying space and resources, but no single microbe guarantees protection.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.7(ii) understand the role of barriers in protecting the body from infection, including skin, stomach acid, and gut and skin flora
Use the science
Why is “take yoghurt, therefore 6.7 is done” the wrong use of flora?
Explore a possible response
The specification asks how resident flora occupy a surface. A food product is not evidence of that mechanism and is not required recall.
Explore the source
- Microbiology Society: what is microbiologyMicrobiology Society
Your skin is occupied territory
6A · Invading the BodyAn NIH-indexed review describes the skin as a physical barrier that also hosts diverse resident microorganisms. Different sites are dry, moist or oily, so they support different communities.…Specification 6.7(ii)Read & use the scienceClose article
Why this matters
An NIH-indexed review describes the skin as a physical barrier that also hosts diverse resident microorganisms. Different sites are dry, moist or oily, so they support different communities. Resident species can compete with incoming microbes for space and resources, although a microbiome is not an impenetrable shield. The distinction matters in advertising: a product promising to “remove all bacteria” ignores that harmless residents are part of normal skin ecology. The specification asks how skin and flora help block entry. It does not require a named brand, a personal skin swab or a claim that every microbe is beneficial.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.7(ii) understand the role of barriers in protecting the body from infection, including skin, stomach acid, and gut and skin flora
Use the science
Why might removal of all resident skin microbes sometimes make colonisation by a pathogen easier?
Explore a possible response
The pathogen may face less competition for space and nutrients, although the physical skin barrier and other defences still matter.
Explore the source
- The human skin microbiomeNature Reviews Microbiology · Abstract