What
is first line of defence in immunology? Physical, chemical and biological
barriers in first line of defence
First
Line of Defence is most basic innate form of non specific defence that prevents
pathogen entry into the body or major organs of the body
1. Physical
barrier
2.
Chemical barrier
3.
Biological barrier
1. Physical barrier
a. Skin:
- is the largest organ of the body and forms the major physical barrier.
- Water proof tough resistant layer formed primarily of keratinocytes. It has protein keratin; that forms a tough layer with other cells like corneocytes.
- Hair follicles traps microbes and skin surface has an acidic pH that inhibits futher colonization
- Continuous replacement of dead skin cells also inhibits bacterial growth
- In regions devoid of skin surface, there are some other physical barriers
b. Nasal
hair: traps microbes along with mucous thus preventing further movement to the
inside
c. Eyelashes
&eyelids: prevents microbes and dusts from reaching the eye surface
d. Mucous
membranes: secreting mucous in mouth, respiratory tract, GI tract and urinary tract.
e. Mucociliary
Clearance: Once the pathogen has entered through the openings; Mucociliary
clearance is an essential part of this defence mechanism. Mucus acts as a
physical barrier, trapping inhaled particles and pathogens, while cilia move
both the mucus layer and fluid out coughing or sneezing. This prevents entry of
pathogen to the lungs from the respiratory tract. The expelled mucus is
then destroyed in the stomach, or coughed up, or sneezed out.This system of
removal is often called the mucociliary escalator. Good mucociliary
escalator is essential for good lung health.
f. Urination:
flushes microbes out of the urethra
There
are physiological responses associated with physical barrier that forcefully
expels microbes out of the system like coughing, sneezing, diarrhea and vomiting.
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2. Chemical barrier
a) Primarily
by maintaining low pH in sites exposed to microbial colonization; thus inhibiting
microbial growth.
- Skin – pH 5.5
- Gastric acid – pH 1-3
- Vagina – pH 4.4
b) Antimicrobial
molecules
Secretory IgA, Lyzozyme, Lactoperoxidase in
Saliva, tears, mucus, colostrum
- Lysozyme, an
enzyme produced in tears, perspiration, and saliva can break down
cell walls and thus acts as an antibiotic (kills bacteria)
- Lactoperoxidase is a peroxidase enzyme secreted from mammary, salivary, and other mucosal glands including the lungs that functions as a natural antibacterial and antiviral agent.
- Sebum secreted by sebaceous glands is an unsaturated fatty acid that provides a protective film on the skin and inhibits growth
- Mucous secreted by mucous membranes is involved in mucociliary clearance mentioned in the physical barrier
- Beta defensins in epithelial cell: secreted from leukocytes and epithelial tissues. They are small proteins (15–20 residues) that function in antimicrobial defense by penetrating a microbe's cell membrane and cause microbial death similar to that of antibiotics.
- Pepsin in gastric mucosal defence: destroying microbial peptides.
3. Biological barrier
Refers to the natural microorganism present in the body; often called as microbiome. Microbiome secretes antimicrobials thus preventing growth of pathogens. Recent research very much highlights the importance of this biological barrier in protecting our body.