The immune system is a collection of mechanisms within our bodies that protects against disease. Disorders in the immune system can result in disease. Diseases occur when the immune system is less active than normal, resulting in recurring and sometimes life-threatening infections.

The immune system is a remarkably effective structure that incorporates specificity, inducibility and adaptation. Failures of host defense do occur, however, and fall into three broad categories: immunodeficiencies, autoimmunity, and hypersensitivities.

Immunodeficiencies occur when one or more of the components of the immune system are inactive. The ability of the immune system to respond to pathogens is diminished with immune responses beginning to decline at around 50 years of age.

Overactive immune responses comprise the other end of immune dysfunction, particularly the autoimmune disorders. Here, the immune system fails to properly distinguish between self and non-self and attacks part of the body.

Hypersensitivity is an immune response that damages the body's own tissues. They are divided into four classes -Type I to IV, based on the mechanisms involved and the time course of the hypersensitive reaction. Allergies, Antibody-Dependency, Hypersensitivity Reactions and Mediated or Delayed type hypersensitivity are the four kinds of hypersensitivities.

Very simply put when your body encounters a pathogen its first line of defense is your immune system. Specialized cells and organs help your body recognize and respond to such elements. Your immune system even has its own circulatory system, called the lymphatic system.