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Innate Immunity: Barriers, Cells, and Mechanisms

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Terms you will need

  • innate immunityInnate immunity is the body's first line of defense against infections, using physical barriers, special cells, and chemical reactions to fight off harmful invaders like bacteria and viruses.
  • anatomical barriersAnatomical barriers are parts of the body like skin and mucus that physically block harmful germs from entering and causing infections.
  • physical barriersPhysical barriers are parts of the body, like the skin and mucus, that stop harmful germs from getting inside.
  • complement systemThe complement system is a group of proteins in the blood that help the immune system destroy harmful germs by making it easier for immune cells to find and kill them.
  • toll-like receptorsToll-like receptors are proteins on immune cells that recognize harmful germs and start the body's immune response to fight them.
  • inflammationInflammation is the body's response to injury or infection, causing redness, swelling, and pain as it tries to heal and protect itself.

How this topic runs

  1. 1Start here
  2. 2How it works
  3. 3At competition depth

The innate immune system is the first line of defense against pathogens in vertebrates and is the dominant immune system in plants, fungi, prokaryotes, and invertebrates. It includes physical, chemical, and biological barriers that prevent the entry of infectious agents. Anatomical barriers such as the skin and mucous membranes form a physical barrier that is impermeable to most pathogens. The epithelial surfaces also help remove infectious agents through shedding, peristalsis, cilia movement, and mucus trapping. Chemical barriers include clotting factors, host defense peptides, and antimicrobial substances secreted by the gut flora. Inflammation is a key response of the innate immune system, initiated by chemical factors released by injured cells. It establishes a physical barrier against the spread of infection and promotes healing. The complement system is another key mechanism of innate immunity, utilizing proteins to identify and eliminate pathogens. White blood cells, including neutrophils, macrophages, and dendritic cells, play a crucial role in phagocytosis, the process by which cells engulf and destroy pathogens. The innate immune system also includes specialized cells such as natural killer cells, which destroy compromised host cells.

Figures (10)

Scheme of the complement system
Neutrophils migrate from blood vessels to the infected tissue via chemotaxis, where they remove pathogens through phagocytosis and degranulation
Inflammation is a process by which the body's white blood cells and substances they produce protect us from infection with foreign organisms, such as bacteria and viruses. The (phagocytes) white blood cells are a nonspecific immune response, meaning that they attack any foreign bodies. However, in some diseases, like arthritis, the body's defense system the immune system triggers an inflammatory response when there are no foreign invaders to fight off. In these diseases, called autoimmune diseases, the body's normally protective immune system causes damage to its own tissues. The body responds as if normal tissues are infected or somehow abnormal.
Phagocytosis versus exocytosis
Membrane attack complex (Terminal Complement Complex C5b-9)
Reaction cascade of the complement system: classical, alternative, and lectin pathways, amplification loop, terminal pathway, and membrane attack complex.
The classical and alternative complement pathways
A flowchart depicting the events of acute inflammation.[21]
Micrograph showing granulation tissue. H&E stain.
Infected ingrown toenail showing the characteristic redness and swelling associated with acute inflammation

Innate immunity is the body’s first line of defense against invaders like bacteria and viruses. This system includes physical barriers like skin and mucus, which block harmful microbes from entering. It also uses chemical defenses, like antimicrobial substances in saliva and gut secretions, to stop infections before they start.

These cells find and destroy invaders through a process called phagocytosis. They engulf pathogens and break them down inside their cells. Other cells, like natural killer cells, target infected or damaged body cells to stop the spread of disease.

It uses proteins in the blood to tag pathogens for destruction and help immune cells respond faster. When tissues are damaged or infected, immune cells release signals that cause redness, swelling, and warmth. This helps trap pathogens and bring more immune cells to the site.

Innate immunity acts quickly and broadly. Together, these defenses form a powerful network that protects the body from harm.

Key Points

  • Innate immunity is the alternate defense strategy of the immune system that is dominant in plants, fungi, prokaryotes, and invertebrates, and functions to recruit immune cells, activate the complement cascade, identify and remove foreign substances, and act as a physical and chemical barrier to infectious agents.
  • Anatomical barriers include physical, chemical, and biological barriers that form the first line of defense against invading organisms, such as the impermeable epithelial surfaces, desquamation of skin epithelium, and the presence of sebaceous glands in the dermis.
  • Physical barriers are part of the anatomical barriers that form an impermeable surface, such as the epithelial surfaces, which prevent the entry of most infectious agents into the body.
  • Chemical barriers are part of the anatomical barriers that include substances such as clotting factors and host defense peptides, which help prevent the survival of microbes by creating an unsuitable environment for them.
  • Mechanical barriers are part of the anatomical barriers that involve physical actions such as the movement due to peristalsis in the gastrointestinal tract or cilia in the respiratory tract, which help remove infectious agents.
  • The complement system is a key mechanism of innate immunity that utilizes proteins to identify and eliminate pathogens, while also maintaining homeostasis by clearing dead and damaged cells through a cascade of reactions that target and kill extracellular pathogens.

Terms

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