Disease and the Body's Defense

A pathogen is an organism that can cause disease. Viruses, bacteria, fungi and parasites are examples of pathogens that can cause infection at a number of different locations in the body.

Once a pathogen infects the body, our defense mechanisms work to destroy it. However, there is always the chance that a pathogen can spread from the initial site. It can multiply and invade surrounding tissues and the bloodstream, causing problems in many parts of the body. Pathogens can also be transmitted from one human to another through the air, via skin-to-skin contact, or on surfaces such as door handles.

You encounter a wide variety of pathogens every day, but your body is protected from them by different defense systems. A naturally existing barrier, such as your skin, is one type of defense against disease. Inflammation is another general defense system that is activated when pathogens enter the body. A third type of defense is the immune response, which is a very specific cellular response to disease.

The human body protects itself with non-specific defense systems that react naturally and automatically to invading organisms. Your skin, with its layers of protective tissue, is a very strong, watertight barrier to environmental pathogens. Another example is in your respiratory tract, where mucus and cilia help trap pathogen particles and move them up to your throat, so you can cough and expel them.

Sometimes pathogens are able to move past natural barriers and enter the body. When body cells are damaged, then inflammation occurs. The signs of inflammation (heat, swelling, redness and pain) reflect the body’s general response to an injury or pathogen.

Inflammation occurs when there is an injury or introduction of anything foreign into your body. When you cut your skin, blood vessels widen to increase blood flow. This enables an explosive increase in the number of white blood cells in the area of injury. These white blood cells, called neutrophils, engulf any foreign material. All of these reactions provide the early stages for wound repair. At the same time, the body’s immune response is also activated.

When challenged by disease or infection, the cells of our body’s immune system go into high gear to protect us from illness. Immune system cells called lymphocytes can distinguish between different kinds of pathogens and have specific reactions to fight against and destroy each type of pathogen.