Body Lines of Defense- Innate and Acquired Immunity


The human body is constantly exposed to various foreign invaders, such as viruses, bacteria, and fungi, that can cause infections and diseases. To protect itself from these harmful agents, the body has developed a sophisticated system of defense mechanisms that can recognize and eliminate them. This system is called the immune system.

The immune system consists of two types of defense systems: innate and acquired. Innate defense systems are present at birth and provide the first and second lines of defense against any pathogen that enters the body. Acquired defense systems are developed during the lifetime and provide the third line of defense against specific pathogens that have been encountered before.

Innate Defense Systems

Innate defense systems are also known as natural or non-specific defense systems because they do not distinguish between different types of pathogens and respond in the same way to all of them. They are composed of physical and chemical barriers, cellular and molecular components, and inflammatory responses that act together to prevent or limit the spread of infections.

Physical and Chemical Barriers

Physical and chemical barriers form the first line of defense against the invasion of pathogens. They include:

  • The skin, which is a tough and waterproof layer that covers the entire body surface and prevents most pathogens from entering.
  • The mucous membranes, which line the internal cavities that are exposed to the external environment, such as the respiratory, digestive, urinary, and reproductive tracts. They produce mucus, a sticky substance that traps pathogens and foreign particles and expels them by coughing, sneezing, swallowing, or urination.
  • The cilia, which are hair-like structures that cover some mucous membranes, such as those in the nose and lungs. They beat in a coordinated manner to move mucus and trapped pathogens out of the body.
  • The tears, saliva, sweat, earwax, gastric juice, urine, and vaginal secretions, which contain various chemicals and enzymes that can kill or inhibit the growth of pathogens. For example, lysozyme is an enzyme found in tears, saliva, and sweat that can break down the cell walls of bacteria. Gastric juice is highly acidic and can destroy most pathogens that enter the stomach through food or drink.
  • The normal microbiota, which are harmless or beneficial microorganisms that colonize various parts of the body, such as the skin, mouth, gut, and vagina. They compete with pathogens for nutrients and space and produce substances that inhibit their growth or stimulate the immune system.

Cellular and Molecular Components

Cellular and molecular components form the second line of defense against the pathogens that manage to cross the physical and chemical barriers. They include:

  • The phagocytes, which are specialized white blood cells that can engulf and digest pathogens and other foreign particles. They include neutrophils, monocytes, macrophages, and dendritic cells. Neutrophils are the most abundant and first to arrive at the site of infection. Monocytes circulate in the blood and differentiate into macrophages or dendritic cells when they enter tissues. Macrophages are large phagocytes that reside in various tissues and organs and can activate other immune cells by presenting antigens. Dendritic cells are phagocytes that capture antigens from tissues and migrate to lymph nodes to activate adaptive immune responses.
  • The natural killer (NK) cells, which are a type of lymphocyte that can recognize and kill virus-infected cells or tumor cells without prior exposure or activation. They release chemicals that induce apoptosis (programmed cell death) or perforate the cell membrane of their targets.
  • The complement system, which is a group of plasma proteins that can be activated by pathogens or antibodies to enhance phagocytosis,