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Barriers to infection (non–specific immunity)

Pathogens are all around us in the environment every day. Soil contains millions of bacteria per cubic centimetre, fungal spores are in the air that we breathe and microbes are deposited when pets or people touch surfaces like tables or kitchen work-tops.

Your body needs to be able to stop these pathogens from gaining entry. This part of the immune system is called non-specific immunity because it is present all the time and not activated in response to a particular pathogen.

Part of body	Bacteria
Head (scalp)	1,000,000 /cm²
Surface of skin	1000 /cm²
Saliva	100,000,000 /g
Nose mucus ('snot')	10,000,000 /g
Faeces	over 100,000,000 /g

Table 1. Concentration of bacteria in and on your body.

Eyes
Tears wash out pathogens and also contain an enzyme that can kill bacteria.

Mouth
Friendly bacteria help to prevent the growth of harmful pathogens.
Saliva cleans and removes bacteria.

Nose
Mucus traps pathogens which are then swallowed or blown out in coughs and sneezes.

Lungs
Mucus in the lungs traps bacteria and fungal spores. Tiny hairs, called cilia, move the mucus to the back of the throat where it is swallowed.

Skin
The outer layer of skin is dead and difficult for pathogens to grow on or penetrate.
Cuts allow pathogens to gain entry to the body.

Stomach
Acid helps to sterilise the food.

Reproductive system
Slightly acid conditions in the vagina and urethra help to stop the growth of pathogens.

Large intestine
Friendly bacteria help to stop the growth of harmful pathogens.
Faeces contains over 30% live bacteria.

Picture 3. Click on the blue spots to see the barriers to infection.

So you think you are clean?
	The table shows the numbers of bacteria that are living on you and inside you all the time. It is easy to see why your body needs barriers to prevent pathogens entering and causing a serious infection.

Picture 5. The structure of the skin.


Skin
	Your skin is possibly the most important barrier to prevent infection entering the body. Cuts and grazes can break this barrier and there are systems to automatically repair any damage. Your skin is a specialised organ that not only protects but also senses the environment and helps to regulate your body temperature. Epidermis The outer layer of the skin. Cells are constantly being made and moved upwards where they form a waterproof, tough, dead surface. Most pathogens cannot penetrate this layer but some fungal infections are able to grow in it. Dead cells are constantly rubbed off and a lot of household dust is made of dead skin cells. Dermis Layer of the skin that contains sensory receptors, capillaries, muscle, hair follicles, connective tissue, sweat and sebaceous glands. It helps to give the skin its structure and strength. Fat A layer of fat helps to store energy and give the body some thermal insulation.

Picture 6. Click on the numbers above to see the steps of repair when the skin is cut.
Open the animation in a new window.

Breaking the barrier

The outer layer of skin forms a tough barrier to infection that is effective as long as it is intact. Cuts, grazes, burns and hypodermic syringes are all ways that this barrier can be broken and pathogens gain entry into the body. To prevent this happening, the skin can repair itself when any damage occurs.

Step 1	Sharp object cuts into skin.

Step 2	Blood enters the wound. A blood clot quickly seals the cut to prevent excessive blood loss and the entry of pathogens. A scab is visible on the surface.

Step 3	New skin grows to repair the damage caused by the cut. The newly-formed skin can sometimes be seen as a scar on the skin surface.