The body has two lines of natural defense to protect against infection. The first are physical barriers, such as the skin, body secretions and mucous membranes. The second line of defense is the immune system. Specialized immune cells coordinate and execute attacks to kill germs that make it past the body's physical barriers.
The skin -- the largest organ in the body -- consists of multiple layers and is the most important natural barrier to infection. The thick outer skin surface prevents most bacteria from passing into the body. Glands in the skin secrete sweat and an oily substance called sebum. Both of these substances inhibit growth of bacteria on the skin surface. Tiny hairs growing from the skin at the entrance to the nose and ears also serve as a natural barriers to infection.
Saliva contains proteins that destroy and prevent bacteria from sticking to the teeth, helping to prevent tooth decay and gum disease. The tear film is the thin layer of tears covering the eyeball. Tear film contains different types of specialized proteins that ward off infections and kill some bacteria. Most germs in food are killed by the strong acid and enzymes secreted by the stomach.
Mucous membranes line the digestive, genital and respiratory tracts and contain glands that secrete mucus. This sticky substance acts as a lubricant but also helps defend the body by trapping bacteria and other particles. Additionally, mucus contains substances that are toxic to some bacteria. The mucous membranes of the airways have tiny hairs on their surface that move back and forth and push mucus and any trapped germs out of the lungs.
Immune System Granulocytes
The immune system comes to the body's defense when physical barriers fail. White blood cells represent an important component of the immune system. One type, known as granulocytes, circulates in the bloodstream and travels to sites of infection in the body. These cells seek out and destroy various types of invading organisms, including bacteria, fungi and parasites.
Immune System Lymphocytes
Lymphocytes are another type of white blood cell in the immune system. These cells are made and stored primarily in the lymph nodes and spleen. There are two kinds of lymphocytes, T and B cells. B cells make specialized proteins called antibodies that circulate in body fluid and tag invading germs for immune system destruction. T cells direct the immune response by communicating with and activating other immune system cells. One type of T cell, called a natural killer cell, attacks and destroys body cells that have been infected to prevent further spread.