Fix the mistake: Immunity

Mechanisms of Biological Defense and Innate Immunity

Our bodies constantly protect themselves from bacteria, viruses, and foreign substances that enter from outside. This mechanism is called biological defense, and its center is immunity. The first defense to function is innate immunity.

Basic Knowledge

The first stage of biological defense is the physical and chemical barriers. The skin prevents foreign substances from entering with its horny layer, while the mucous membranes trap them in mucus(mucin). Tears, sweat, and saliva contain lysozyme, which has a bactericidal effect, and the stomach contains strongly acidic gastric acid(pH1〜2), which kills most pathogens.
When pathogens pass through the barriers, innate immunity functions. Its main cells are phagocytes: neutrophils・macrophages・dendritic cells. They take in and decompose foreign substances through phagocytosis. In addition, NK cells(natural killer cells) directly attack virus-infected cells and cancer cells. Innate immunity functions immediately after invasion and within several hours, and it responds broadly regardless of the target.

Important Terms
Biological defense（A series of mechanisms that protect the body from pathogens and foreign substances）
Physical and chemical barriers（Skin・mucous membranes・lysozyme・gastric acid）
Innate immunity（Immediate, nonspecific immunity present from birth）
Phagocytes（Neutrophils・macrophages・dendritic cells）
Phagocytosis（The function by which phagocytes take in and decompose foreign substances）
NK cells（Lymphocytes that directly destroy virus-infected cells and cancer cells）
Inflammation（Redness・heat・swelling・pain occurring in injured tissue）

Further Study

Innate immunity is an ancient system that has evolved for more than about 500 million years and is also found in plants and insects. Phagocytes recognize molecular patterns common to pathogens(such as components of bacterial cell walls) through pattern-recognition receptors(TLR). Rather than responding specifically to one antigen, they detect things collectively as “bacteria-like” or “virus-like.”
The inflammatory response in which a wound becomes red, swollen, and hot occurs because histamine and other substances dilate capillaries and cause white blood cells to accumulate. Fever is a defense response that suppresses pathogen growth and increases the activity of immune cells. After phagocytosis, dendritic cells move to the lymph nodes and serve as a “bridge” that activates adaptive immunity.

Key Points
  • First barrier=physical and chemical barriers of skin・mucous membranes・lysozyme・gastric acid
  • Second barrier=innate immunity(phagocytes・NK cells・inflammation)
  • Phagocytes=three types: neutrophils・macrophages・dendritic cells
  • Phagocytosis=take in and decompose
  • NK cells=destroy infected cells・cancer cells
  • Innate immunity is immediate(several minutes〜several hours)・nonspecific
  • Dendritic cells serve as a bridge to adaptive immunity

Points to Note

① Innate immunity is nonspecific(responds regardless of the target), while later adaptive immunity is specific(responds to each target). ② NK cells are a type of lymphocyte but are classified as innate immunity cells(not adaptive immunity). ③ Distinguish phagocytosis from a phagosome. A phagosome is an intracellular vesicle formed by phagocytosis. ④ Inflammation is a defense response, not harm. The “four cardinal signs”—redness, heat, swelling, and pain—are classical.

Practice

  1. Name all three types of phagocytes responsible for innate immunity.
  2. What enzyme with a bactericidal effect is contained in tears and saliva?
  3. What lymphocytes directly destroy virus-infected cells?

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