The first lines of defense of the human immune system include physical barriers like the skin and mucous membranes. The skin forms a protective barrier against pathogens, while mucous membranes secrete mucus to trap and remove microbes via cilia. Hair also provides a physical barrier by blocking particles from entering moist areas. Various secretions like tears, saliva, urine, and perspiration continually rinse surfaces to prevent microbial colonization. Beneficial microflora that normally colonize surfaces compete with and inhibit pathogens from colonizing through various chemical methods.

1. 16.2 First Lines of Defense
List and describe the physical factors of the first
line of defense of the human immune system.
a) How do skin and the mucous membranes
provide immunity?
b) How do tears, saliva, urinaCon, perspiraCon
and vaginal secreCons provide protecCon?
c) How is the protecCon provided by hair (nose
hairs, eyelashes) different from that provided
by cilia?

2. 1st Lines of Defense:
Skin & Mucous Membranes
• Mechanical protecCon ‐ Barrier
– skin (epidermis) closely packed,
keraCnized cells
• shedding helps remove
microbes
• Hard to penetrate through
– mucous membrane secretes
mucus: trap &
move microbes
as a “ciliary
escalator”

3. 1st Lines of Defense: Hair
• Mechanical protecCon – Barrier
• Nose, eyelashes, eyebrows, ear
• Block entry by parCcles into the moist environments
of the eye, outer ear and respiratory tract
Ear hair champion!

4. 1st Lines of Defense:
SecreCons ‐ Rinsing
• Tears, vaginal
secreCons, perspiraCon
& saliva conCnually
rinse or wash surfaces
to prevent microbes from colonizing
• Expelling of materials by defecaCon, urinaCon,
emesis, sneezing, coughing achieves a similar
effect

5. 1st Lines of Defense:
SecreCons ‐ Chemical
• Some secreCons have a low pH that
creates an inhospitable environment
for many microbes
– Gastric juice: pH 3
– Vaginal secreCons/lacCc acid: pH 4.0
– Urine: pH 6
• Lysozyme is an enzyme present in saliva, tears, urine,
nasal mucus, sweat
– Enzyme that metabolizes pepCdoglycan
– PepCdoglycan is a component of bacterial cell walls
– More effecCve against Gram posiCve

6. 1st Lines of Defense:
Microflora
• 10x as many microbial cells colonizing our
surfaces as there are human cells in the enCre body
• Skin & hair
– Harmless microbes feed on sebum, other secreCons & dead skin
cells
– Acinetobacter johnsonii, Staphylococcus aureus, Candida
albicans, Demodex folliculorum
– CompeCCon for territory excludes pathogens
• GI tract
– Lactobacillus acidophilus, Escherichia coli, Bifidobacterium
bifidum
– Acids & anCmicrobial chemicals released to prevent pathogens
• Vagina
– Lactobacillus acidophilus
– Produces lacCc acid as a result of glycogen metabolism