how does skin help defend the body against infections?

Answer 1

Skin provides protection from foreign invaders in several ways. Apart from the physical barrier there are specialised cells of the immune system throughout the layers of the skin. Some of these cells detect invasion by foreign proteins such as bacteria or viruses and other cells have the function of destroying and removing such material.

When a foreign protein (called an ‘antigen’) comes into contact with cells of the immune system these cells produce proteins (called antibodies) that fit around the invading antigen in a unique way. A fair analogy is of a lock and key, in which the invader is the key and the immune system cells manufacture a lock with which to trap it. The antibody-antigen combination is recognised by other cells within the immune system family, which then move in and destroy the invader.

In the type of immune reaction called ‘hypersensitivity’, the presence of foreign material sets off reactions from skin cells called mast cells. These contain powerful signalling chemicals such as histamine which, when released, activate the other components of the immune system.

Histamine has marked local effects on the skin, familiar to anyone who has brushed against stinging nettles. At the contact site the skin swells and becomes red, due to opening up of the blood vessels and leakage into the tissues of fluid from within the blood vessels and the lymphatic system.

Surrounding the contact site the skin blood vessels contract, restricting the flow of blood and so causing the skin here to pale (the so-called ‘wheal and flare’ reaction). Such a hypersensitivity reaction occurs almost immediately after being pricked by stinging nettles, hence the common term ‘nettle rash’. The proper medical term is urticaria and a great many other agents apart from nettles, such as chemicals and food components, can trigger similar reactions.

The ‘nettle rash’ form of sensitivity is the easiest to understand but there are several other ways in which immune reactions occur within the skin. These usually act over longer time scales. In allergic contact dermatitis for example it takes two or three days for the immune system cells to recognise the presence of the irritation and to recruit more cells locally to deal with the situation. Such a delay makes it harder to work out what caused the allergic reaction in the first place.

Answer 2

The same way a raincoat protects you from the rain.

Answer 3

By preventing them from getting directly into your blood and organs? It also keeps your guts from spilling onto the dirty, dirty ground.

Answer 4

it has bacteria on it that keep other bacteria and viruses from entering sweat also helps out

Answer 5

Think of it as a filter