Virtual Medical Centre

Innate Immune System

• Introduction to the innate immune system
• Physical barriers to infection
  • Skin
  • Lungs
  • Eyes
  • Mouth
  • Stomach
  • Urinary tract
  • Vagina

Introduction to the innate immune system

The body is sometimes described as constantly being 'at war'. Everywhere around us there are dangerous bacteria, viruses, parasites and toxins that can cause the human body harm. To combat this, the body uses the immune system to fight away anything that may cause damage. The innate immune system are those parts of the immune system that work no matter what the damage is caused by, and are all aimed at protecting the body without the need for a lot of preparation. They are always at work and do not need to have seen the offending invader before to be able to start attacking it.

Physical barriers to infection

The major defences of the innate immune system are those that stop the infection from getting into the body in the first place. If they can do this, then it makes killing the few that get through a much easier task for the body.

Skin

The skin is a thick barrier to infection and protects from bad bacteria in a few ways.

1. It has a lot of dead skin cells that block the entry of bacteria
2. The skin has its own, perfectly healthy, population of bacteria. These compete with the bad bacteria to stop them from multiplying.
3. Sweat glands and sebaceous glands (that secrete a thick, oily substance called sebum) both release a slightly acidic substance onto the skin, slowing the growth of bacteria.

If the skin is constantly damp, such as the skin between toes, under arms or even under rolls of fat, it weakens the barrier. This wet skin can be more easily broken by the fungus that usually live on the skin and allows them to grow, causing a fungal infection. If the skin is broken by a cut or graze then it allows a far more easy access to the body than otherwise and these areas can get infected more easily. During surgery, even though every attempt to stop infection is made, sometimes the body's own normal bacteria can cause an infection because of the cut.

For more information, see Anatomy of the Skin.

Lungs

The lining of the airways of the respiratory system are also coated in a thick, mucous-like substance that acts as a protective barrier. These airways are basically like big pipes, with lots of air flowing through all the time. With this air comes many possibly damaging things so the lungs must be protected. In the lungs, as air rushes past the walls, bacteria adhere to sticky mucus. In this mucus is an enzyme, which is the name for a substance that causes a chemical reaction. Here, the enzyme, which is called lysozyme, causes the bacteria to be broken down into its basic parts. It is being treated much like a food, and is being digested by this enzyme. After it has been digested, the remains of the dead bacteria as well as other substances are swept out of the air ways by small hair-like projections called 'cilia' which line the walls.

For more information, see Anatomy of the Respiratory System.

Eyes

Tears act to stop infection in two ways. Firstly, the tears physically wash away a lot of bacteria. Secondly, tears also contain the same enzyme that was mentioned before, lysozyme, which protects the eye from bacteria.

For more information, see Anatomy of the Eye.

Mouth

The mouth is full of highly dangerous bacteria that can easily destroy tissue and cause tooth decay. Saliva helps prevent the destructive processes in several ways:

1. The flow of saliva helps wash away bacteria, as well as the food particles that the bacteria eat.
2. The saliva contains substances, including lysozyme and others, that work to destroy bacteria.
3. Lysozyme also destroys the bits of food that the bacteria use for food.
4. A large amount of protective antibodies (which are substances that recognise infective particles) are found in the saliva.

Stomach

The stomach is a highly acidic environment, which is very destructive to most types of bacteria. Also, if a bacteria or other dangerous substance gets into the stomach, then they are treated just like any type of food and often broken down and destroyed by digestive enzymes before they can cause any damage.  

For more information, see Anatomy of the Gastrointestinal System.

Urinary tract

The urine is slightly acidic, meaning bacteria do not grow very well. The frequent flushing action of urination also helps to lower the amount of urinary infections.

Vagina

In a similar way to the skin, mucous linings of the vagina help form physical barriers to the entry of infective organisms. Also like the skin, the vagina is home to a lot of native bacteria that compete with the dangerous types and stop them from growing. Also in the vagina, there is a population of a type of bacteria called Lactobacillus that produces an acidic substance called lactate, helping to keep the environment slightly acidic and slowing bacterial growth.

For more information, see Anatomy of the Female Urogenital System.