Three lines of defence
Pathogens such as bacteria and viruses constantly try to enter the body. Our immune defences work in three lines, the first two being non-specific (they act against all invaders) and the third being specific.
First line: barriers
These stop pathogens from entering:
- Skin — a tough physical barrier.
- Mucous membranes — mucus in the nose and airways traps microbes; cilia in the trachea sweep the mucus away.
- Stomach acid — its low pH kills many swallowed microbes.
- Lysozyme — an enzyme in tears and saliva that destroys bacteria.
Remember
The first line is about keeping pathogens out; it does not target any specific microbe.
Second line: non-specific response
If pathogens get past the barriers, white blood cells called phagocytes destroy them by phagocytosis — they engulf and digest the microbes. Damaged tissue also becomes inflamed, bringing more phagocytes to the area. The pus that forms at an infected wound is largely made of dead phagocytes and destroyed microbes.
Third line: specific response
The third line is carried out by lymphocytes. A foreign molecule that triggers this response is called an antigen. B lymphocytes produce antibodies that bind precisely to a particular antigen, while T lymphocytes help coordinate the response and destroy infected cells. This line also creates memory cells for faster future protection. Because the third line is specific and takes time to build up, the first two lines are important for slowing the infection down and buying time.
Together the three lines give overlapping protection: the barriers keep most microbes out, phagocytes quickly deal with those that get in, and lymphocytes provide a targeted, long-lasting response.
Example
When you cut your finger, phagocytes gather at the wound (second line) to engulf any bacteria that got past the skin (first line) before lymphocytes launch a specific attack (third line).