1. Body’s Defence
Mechanism
You must be able to :
Identify the 3 lines of defence mechanism
Describe phagocytosis
State the meaning of antigen & antibody
State the meaning of immunity &
imunisation
Relate antigen & antibody to imunity
Name & give examples of various types of
immunity

2. Body’s Defence
Mechanism
1st Line of
Defence
3rd Line of
Defence
2nd Line of
Defence

3. 1stentering Defence by mean of physical &
Prevention of pathogens Line of the body
chemical barriers
A non-specific defence, that is never differentiate among various type
of pathogens. This includes:
i) Skin
As a physical barrier, skin is made up of a dead keratinised layer,
tough enough for pathogens to penetrate
If there is a scratch or cut, the blood clots to seal the wound & avoids
infection
Also acts as chemical barrier as it secretes sweat which contains salt.
Sebaceous glands produce sebum which contains acid & oil. All these
substances are unfavourable for growth of microorganisms
Sweat also contains lysozyme which destroys pathogens
ii) Tears & Saliva
Contain lysozymes which protect the eyes & mouth from pathogen invasion.

4. 1st Line of
Defence
iii) Gastric juice in stomach
Contain hydrochloric acid which destroys most pathogens in foods &
drinks taken.
iv) Mucous membranes
Secrete mucus in nasal cavity & trachea to trap the dust particles & spores
The cilia in the respiratory track sweep the trapped particles to the
pharynx & stimulates sneeze @ cough to expel out the pathogens.

5. The killing action brought by some of the white blood cells
like neutrophil & monocyte (Phagocytes). The process is
called phagocytosis.
Phagocytosis occur when pathogens get through the first line
of defence. Phagocytes move to the infected area due to the
stimulation by chemicals released by damaged cells, example
cut skin.
Sometimes the phagocytes are killed by toxins produced by
the pathogens.
Dead bacteria, tissue cells & phagocytes may accumulate to
form pus at the site of injury.
2nd Line of
Defence
Phagocytosis

6. 3rd Line of
Defence