First line of defence

The first line of defence is involved in  non-specific responses to pathogens to prevent them from entering the body. This type of defence against infection involves body barriers and chemical secretions.

Human first line of defence:

Intact skin:

This provides a barrier of entry of pathogens as long as the skin is intact. Any cut or abrasion in the skin will damage the skin’s shielding properties and allow pathogens access. Skin also has chemical protective mechanisms, as glands in the skin can secrete fatty acids and sweat containing salts that inhibits bacteria.

Mucous membranes:

Mucus is secreted by cells lining the respiratory tract and helps trap bacteria and then sweeps them up the throat with the action of cilia, allowing the individual to cough them up and remove them, or blow them out of your nose through sneezing. Mucus also exists in the digestive tract forming a protective barrier particularly in the stomach. It is also present in the vagina to trap  bacteria preventing them entering the genito-urinary tract.


Saliva present in the mouth contains lysozymes that cause bacteria to lyse or burst


Tears like saliva contain lysozymes that cause bacteria to lyse or burst.

Stomach acid:

The stomach is kept at a very low pH due to the hydrochloric acid present. The acidic environment  creates unfavorable living conditions for a lot of bacteria and kills them.

Plant defences:

Plants do not contain an immune system, but they still have nonspecific responses to prevent themselves from infection through various adapted structural features and chemical secretions.

These  mechanisms include:

  • Hairs on leave and stem surfaces to prevent pathogens contacting plant tissue.
  • Waxy  and thick cuticles and thick barks to prevent pathogen entry.
  • Formation of galls which are impenetrable capsules capable of trapping the pathogen.
  • They are able to deter organisms through the action of resins, tannins and phenolic substances due to their toxic effects on pathogens.
  • The silicon content in some leaves makes the plant resistant to degrading enzymes released by pathogens.
  • Some plants such as lemon trees and mint plants are able to produce oils that repel insect pests.
  • Stone fruit trees such as plum and peach are able to secrete gum around an infected area to ‘seal it off’ preventing from it from coming in contact with the plant.

See also