First Line of Defense in Immune System

 The First line of defence is the part of Innate immune system. The function of first line of defense is to prevent pathogens from entering the body. The first line of defense is formed by skin and mucous membranes (of respiratory system, gastrointestinal system, urinary tract, reproductive system).

The Frst line of defense is formed by skin and mucous membranes and they provide physical and chemical barriers to pathogens


Skin is the body organ with the greater surface area and it provides the major barrier to pathogens. Skin consists of two parts the outer part is known as epidermis and the deeper part is known as dermis.


Skin-Epidermis is the outer thinner portion of the skin, thus it is in direct contact with the external environment. Epidermis consists of closely packed cells which are known as keratinocytes.

  Keratinocytes produce keratins which are insoluble protein and the resistance to bacterial enzymes and toxins. thus they provide defense against bacteria.

  Outer skin cells shed continuously. This shedding removes microorganism that adhere to the surface of the skin.
Skin has low pH which discourages growth of many microbes.

Skin - Dermis

Dermis is the inert thicker portion of the skin which is composed of connective tissue. Dermis contain sebaceous or oil glands which produce oily substance known as sebum. This sebum contains unsaturated fatty acids which inhibit growth of certain bacteria and fungi.

Dermis also contain a sweat gland which produce perspiration.  Perspiration flushes microbes form the surface of the skin. Perspiration also contains salt, antimicrobial peptides and Lysozyme. All these are antimicrobial in action.

  For example salt creates osmotic action withdraw water from the bacteria or microbe. This inhibit their growth and kill them. Lysozyme is a bacteriocidal  in action, which breaks down their cell walls by attacking the peptidoglycan. 

Mucus membranes 

  Mucous membrane or mucosa is a membrane that lines reduce cavities in the body and surrounds internal organs, that means it lines the respiratory, urinary, digestive and reproductive tracts.

  One important feature of nuclear membrane is that the surface cells of mucous membranes are continuously shed and replaced by steam cells. This shedding process removes the attached microbes.

  Mucus membranes are moist. This is because they secrete a thick fluid known as mucus.
The function of mucus is to lubricate and moist in the cavity surface. Besides that this mucus also traps the pathogens and dirt which is expelled or eliminated by the body.

The Upper Respiratory Tract

  In the Upper respiratory tract of human body; Nose contains mucus coated hairs. These mucus coated hairs trap and filter microbes from the air during inhalation. The mucus of the nose also contains lysozyme which is bacteriocidal in action.

  Other than that mucous membrane of the upper respiratory tract contains cilia which are microscopic hair like projections. These help to propel inhale and trap dirt and microbes towards the throat. Sneezing and coughing are the reflections which clear our respiratory system.


If you look at the ear of human body and there are two features which are a difference barrier to pathogens these are hairs in the external ear and ears wax (Cerumen).

  Both these feature prevent the entry of microbes inside the ear. Earwax is a mixture of secretions which are rich in fatty acids that are responsible for the low pH of the ear and this low pH inhibits the growth of many pathogenic microbes.

Human Eye

  the lacrimal glands of the human eye secrets these tears keep congestive of the eye moist and this prevents the microbes from setting on the surface of eye.
Tear also contains lysozyme which is antimicrobial in action.


Saliva is produced by salivary glands and they also contain antimicrobial substances like lysozyme. Besides that flow and washing action of saliva helps in checking the microbial colonization in the mouth.
  pH of saliva is in between 6.4 - 6.5, which is also inhibitory to microbial growth.


Epiglottis is a small part of cartilage which covers larynx during swallowing. This prevent the entry of microbes in the lower respiratory tract.


Stomach contains gastric juice. This gastric juice contains HCL (hydrochloric acid), enzymes such as pepsin and mucus.
The pH of stomach is in between 1.2 - 3. The very high acidity of stomach is capable of destroying the area and most bacterial toxins.

Urinary tract

Urethera is cleaned by flow of urine. Urine contains lysozyme and it has acidic pH that is average 6 so these features inhibit microbes and their colonization in the urinary tract.

Reproductive tract

  In the female reproductive system the vaginal secretions result in acidic pH which is between 3 & 5. this pH is inhibitory for the growth of microbes other than that menstrual flow cream uterus and vagina.