Pathogenic organisms need to enter the body of their host before they can cause harm. Evolution has selected hosts adapted to defend themselves against such invasions. The mechanisms that have evolved to prevent entry of pathogenic organisms are called primary defence.
The body is covered by the skin. This is the main primary defence. The outer layer of the skin is called the epidermis. The epidermis consists of layers of cells. Most of these cells are called keratinocytes. These cells are produced by mitosis at the base of the epidermis. They then migrate to the outer surface of the skin. As they migrate they dry out and the cytoplasm is replaced by the protein keratin. This process is called keratinisation. It takes about 30 days. By the time the cells reach the surface, they are no longer alive. Eventually the dead cells slough off. The keratinised layer of dead cells acts as an effective barrier to pathogens.
Certain substances, such as oxygen and nutrients in our food, must enter our blood. This leaves the body exposed to infection, as the barrier between the blood and our environment is reduced. In areas such as the airways, lungs and digestive system, there is great potential for infection. We take air and food in from our environment, and these materials could be harbouring many microorganisms.
These areas are protected by mucus membranes. The epithelial layer contains mucus-secreting cells called goblet cells. In the airways, the mucus lines the passages and traps any pathogens that may be in the air. The epithelium also has collated cells. The cilia are tiny, hair-like organelles that can move. They move in a coordinated fashion to waft the layer of mucus along. They move the mucus up to the top of the trachea, where it can enter the oesophagus. It is swallowed and passes down the digestive system. Most pathogens in the digestive system are killed by the acidity of the stomach, which can be pH 1-2. This denatures the pathogens enzymes.
Mucous membranes are also found in the gut, genital areas, anus, ears and nose.
Other primary defences:
· The eyes are protected…