Besieged Castle: The Human Body

It is a fact that even though we try to live in clean environments, we share these places with many micro-organisms. If you had the chance to view the room you are currently sitting in with a microscope, you would immediately see the millions of organisms you live with. 

In this situation, the individual resembles a "besieged castle". Needless to say, such a castle, which is surrounded by countless enemies, must be protected in a very complete and organized manner. Human beings are created along with this perfect protection they need, and are not, therefore, entirely defenceless against these enemies. The "micro" guards in our bodies never leave us alone and fight for us on many fronts. 

The invader cells that want to take control of the body first have to fight their way through the front line of the body. Even though these fronts have their weaknesses at times, the enemy is hardly ever allowed to pass through them. The first front the enemy must penetrate is our skin. 

The Protective Armour of Our Body: The Skin

The skin, which covers the entire body of a human being just like a sheath, is full of amazing features. Its ability to repair and renew itself, its non-permeability by water, despite the existence of tiny pores on its surface as opposed to its function of discharging water through perspiration, its extremely flexible structure, allowing free movement, as opposed to its being thick enough to avoid easy rupture, its ability to protect the body from the heat, the cold, and harmful sunrays are only a few of the features of the skin that have been specially created for human beings. Here, we will deal with a particular feature of this extraordinary wrapping paper: its ability to protect the body from disease-causing micro-organisms. If the body is considered a castle besieged by enemies, we can safely refer to the skin as the strong walls of this castle. 

The main protective function of the skin is realized via the dead cell layers constituting the outer section of the skin. Each new cell produced by cell division moves from the inner section of the skin towards the surface. While doing this, the liquid element (cytoplasm) of the cell interior transforms into a resistant protein known as keratin. During the process, the cell dies. The newly formed keratin substance has a very hard structure and is not therefore subject to decomposition by digestive enzymes, which is a sign of its resistance. Thus, invaders such as bacteria and fungi will be unable to find anything to rip off from the outer layer of the skin. 

Moreover, dead outer cells containing keratin are constantly shed from the skin surface. The new cells that come from beneath to replace the discarded ones form an impenetrable barrier in that area. 

The organisms on the skin fulfill another protective function of the skin. A group of harmless microbes live on the skin, which have adapted to its acidic medium. Feeding on the leftovers stuck on the keratin of the skin, these microbes attack all kinds of foreign bodies to protect their feeding site. The skin, as the host of these microbes, is like a supplementary force that provides external support to the army within the human body. 

The first defense response of the organism against its dangerous invaders is the rapid self-repairing of the skin tissue following the infliction of a wound. When such a wound ruptures the skin, defence cells immediately travel to the injured area to fight with the foreign cell and to remove the debris of the affected tissue. Later, some other defence cells enhance the production of fibrin, which is a protein that rapidly re-covers the wound with a fibrous network. This picture is of a fibrin that has spread over some red blood cells.

AN IN-DEPTH VIEW OF THE SKIN
Above is a cross-section of the skin. The sweat droplets secreted from the skin play a variety of roles in the body. In addition bringing down the body temperature, they provide nutrition for certain bacteria and fungi living on the surface of the skin, and produce acidic waste materials such as lactic acid which helps decrease the PH level of the skin. This acidic medium on the skin surface creates a hostile environment for any harmful bacteria that are looking for a place to live.  

This is a close-up of the sweat gland entrance. Here, too, you will find bacteria just like everywhere else on the skin. This picture, which is magnified 5900 times, shows the cells in the trachea (blue). They use their glands (yellow) to secrete a substance that traps the particles in the air. Above, you can see the macrophages located in the lung tissues. They eliminate the dust particles in the air we inhale.xxxxxxxxxx .