Immune System



The immune system is the body's system of defense against pathogens and diseases that could harm the body. The first line of defense includes physical barriers such as the skin and mucous membranes that prevent pathogens from entering the body. The second line of defense provides an immediate, nonspecific response, attacking pathogens indiscriminately. The second line of defense includes phagocytic cells as well as biochemical processes such as inflammation and fever. The body's third line of defense is slower but produces a response that is specifically targeted toward a particular pathogen. This third line of defense can also "remember" pathogens that it has encountered and is thus described as adaptive or acquired immunity.

At A Glance

  • The first line of defense prevents pathogens from entering the body, the second line of defense attacks pathogens indiscriminately once they do enter the body, and the third line of defense attacks and remembers pathogens.
  • There are five types of leukocytes, also called white blood cells—neutrophils, monocytes, eosinophils, basophils, and lymphocytes—with each having a specialized mechanism to fight foreign invaders.
  • As a first line of defense, the skin and mucous membranes provide a physical barrier to pathogens and also secrete chemicals that can destroy or inhibit pathogens.
  • Inflammation is a secondary line of defense feedback mechanism to injured tissue that is activated after phagocytic cells, the body’s first line of immune defense, has responded, resulting in increased blood flow to the site and mobilizing the defensive cells.
  • As a third line of defense, cell-mediated immunity—part of adaptive immunity—involves various types of cells that fight a specific pathogen and recognize antigens, which are molecules that trigger a response from the adaptive immune system, for example, surface proteins of bacteria or viruses.
  • Active natural immunity occurs when the body produces antibodies after natural exposure to an antigen, and passive natural immunity occurs when antibodies are passed from mother to offspring.