General Features of the Human Immune System

In addition to your body's "average Joe working cells", you are internally patrolled by a microscopic police force; the cells and molecules of your immune system. These cellular cops are constantly on the lookout for bad guys, finding interlopers and eliminating them. Bad guy microbes (pathogenic bacteria, fungi, protozoans & viruses) are everywhere. You can't avoid them, and your body's police force is constantly intervening to prevent you from being colonized by these pathogens.

Nonspecific Immune Defenses

Nonspecific immune defenses are general ways in which the body prevents and combats infection; tactics that do not target or recognize specific microbes, but deter pathogens none the less. The human body has two nonspecific lines of defense against pathogens.

The First Line of Defense: The first line of immune defense has both physical and chemical components, and consists of the skin, mucous membranes and their products. Your skin and mucous membranes are physical barriers that produce chemicals which deter microbes, such as salty sweat and oily sebum. These barriers and chemicals are all part of an external fortress that helps prevent pathogens from getting inside the body in the first place.

The Second Line of Defense: The second line of nonspecific immune defense comes into play when pathogens succeed in penetrating the skin or mucous membranes. The players in nonspecific defense consist of cells, antimicrobial chemicals, and processes, but not physical barriers. Many components of second line defense are contained, or originate, in the blood, including leukocytes (white blood cells).

Elements of the second line of immune defense include:

• Phagocytosis: Leukocytes that can engulf and destroy pathogens.
• Extra-cellular killing: Leukocytes that secrete chemicals that can destroy microbes.
• Nonspecific chemicals, such as defensins, interferon and cytokines that help regulate immune response.
• Inflammation: Swelling at the site of tissue damage that helps attract leukocytes to the compromised area.
• Fever: A change in normal body temperature that generally enhances the immune system and deters pathogens.

Specific or Acquired Immunity - The Third Line of Defense

Your body's third line of immune defense is a "smart system" that can learn and change. Specific immunity involves cells and cell products that can recognize and remember specific pathogens. This is why a person sometimes becomes ill with an infectious diseases one time only, and never again. This is because, after initial exposure, the body has a police squadron of cells specifically trained to find and destroy that particular pathogen, if that pathogen ever enters the body again. Specific immunity is also why vaccinations can be used to prevent illness.

A vaccination usually contains an either dead or weakened microbe, or portions of a microbe, that can no longer cause disease. Intentionally exposing the body to this weakened pathogen is kind of like showing the cells of our specific immune system a "wanted poster", so that if they run into this bad microbe in the future, they can identify and eliminate it.

Sources

Bauman, R. (2004) "Microbiology." Pearson Benjamin Cummings.Tortora, G.; Funke, B.; and Case, C. (2010) "Microbiology: An Introduction." Pearson Benjamin Cummings.