11.1 - Defence Against Disease: Barriers to Pathogens
- 1The difference between non-specific and specific defence mechanisms
- 2The physical and chemical barriers to pathogens
- 3What antigens are
Non-specific and specific defence mechanisms
The body has a number of defence mechanisms to protect itself from pathogens.
These defence mechanisms can be split into two groups:
- Non-specific defences - These act quickly to defend the body, but respond in the same way for all pathogens.
- Specific defences - These are slower to defend the body, but produce a specific response for each pathogen.
Physical and chemical barriers
The body has a number of non-specific barriers to prevent the entry of pathogens.
Skin:
- The skin acts as a physical barrier to block pathogens from entering the body.
- It also acts as a chemical barrier by producing sebum, an oily, antimicrobial substance that lowers pH to inhibit the growth of pathogens.
Mucous membranes:
- Parts of the ears, nose, throat, and digestive tract are lined by mucous membranes.
- These membranes secrete mucus to trap pathogens and use lysozymes to destroy them.
Expulsive reflexes:
- Coughing and sneezing are methods for expelling foreign objects (or pathogens) from the gas exchange system.
- Vomiting and diarrhoea expel the contents of the gut along with any pathogens present.
Blood clotting and wound repair:
- A cut to the skin provides a possible entry for pathogens and so blood clots act quickly to seal any wounds.
- The clot dries out to form a scab that blocks entry to the body.
- After a scab has formed, the skin is capable of repairing itself to reform its physical barrier.
- Epidermal cells underneath the scab divide while damaged blood vessels regrow and collagen fibres are used to provide strength to the new tissue.
- Once the epidermis is the required thickness, the scab breaks off and the wound is healed.
Inflammation
Inflammation consists of swelling, heat, redness, and pain at the site of infections or wounds.
It can be triggered by damaged tissues which release chemicals affecting blood vessels in two ways:
- Blood vessels dilate which increases blood flow to the area, making it hotter to prevent pathogens from reproducing.
- Blood vessel walls become more permeable so that they start to leak tissue fluid, causing swelling and isolating any pathogens in the damaged tissue.
Antigens
Antigens are unique molecules (usually proteins) that can be found on the surface of cells.
These antigens allow the immune system to distinguish between the body's own cells (self) and foreign cells (non-self). Any foreign cells can then be destroyed whilst leaving the body's own cells unaffected.
Antigens allow the immune system to identify various cells and molecules:
- Pathogens - The immune system recognises antigens as being foreign and activates cells to destroy the pathogen.
- Abnormal body cells - Cancerous or infected cells display abnormal antigens that trigger an immune response.
- Toxins - These are antigen molecules themselves (not cells) and can be recognised by the immune system.
- Cells from other organisms of the same species - These cells may have different antigens to the body's own cells and so are identified as being foreign. This can cause the rejection of transplanted organs.