Barriers and innate immunity

We live surrounded by micro-organisms, and we carry more of them than we have cells. Yet we rarely fall ill: a defence system constantly sorts what belongs to the organism from what is foreign to it.

Barriers, the first line

Before any reaction, there are physical and chemical obstacles:

SKIN            mechanical barrier, dry and acidic (pH ≈ 5.5)
MUCOSAE         mucus that traps, cilia that sweep out
TEARS, SALIVA   contain lysozyme, an enzyme that destroys bacteria
ACIDITY         the stomach at pH 2 sterilises most of what is swallowed
FLORA           commensal bacteria occupy the space and the resources

The flora deserves special mention: it protects simply by occupying the ground. This is why antibiotic treatment, which destroys it along with the rest, favours opportunistic infections — thrush, or Clostridium difficile colitis.

Recognising non-self

Any defence requires a sorting ability:

SELF       the organism's own cells, carrying its markers
NON-SELF   bacteria, viruses, grafted cells, cancer cells

Body cells display surface markers, the MHC (major histocompatibility complex), which work like a molecular identity card. Anything without them — or with different ones — triggers the defence.

Innate immunity: fast and non-specific

INNATE IMMUNITY
   present from birth
   acts within MINUTES
   treats every intruder THE SAME WAY
   keeps NO memory

Its main actor is the phagocyte, a cell able to engulf and digest an intruder:

   1. adhesion        2. ingestion       3. digestion      4. expulsion
       ●                  (●)               (·)              ○
     [cell]             [cell]            [cell]           [cell]

This mechanism, phagocytosis, was described by Metchnikoff in 1882 — he observed it by pricking a starfish larva with a rose thorn and seeing mobile cells surround the foreign body.

The inflammatory response

This is the visible manifestation of innate immunity. Four signs, described since antiquity:

REDNESS   \
HEAT       |   vasodilation: more blood arrives at the site
SWELLING   |   plasma and cells pass into the tissues
PAIN      /    substances irritating nerve endings

Contrary to intuition, inflammation is not the disease: it is the response. Redness and swelling signal the influx of defences. Systematically suppressing it with anti-inflammatories brings relief but can also slow the resolution of the infection.

Likewise fever is a defence mechanism: it slows the multiplication of many micro-organisms and speeds up immune reactions. Moderate fever need not be fought at all costs.

Summary

• Barriers (skin, mucosae, acidity, flora) block most intruders.
• Defence rests on the self / non-self distinction, via MHC markers.
• Innate immunity is immediate, non-specific and memoryless.
• Phagocytosis engulfs and digests intruders.
• The inflammatory response (redness, heat, swelling, pain) is a response, not the disease.
• Fever is a defence mechanism, not merely a symptom.