What is First Line of defence in immune system? Physical, Chemical & Biological barriers

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

First Line of defence in immune system

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

Physical barrier - First line of defense

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.


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

Chemical barrier - First line of Defence

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.

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