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Innate Immunity: Barriers, Cells, and Mechanisms

The innate immune system or nonspecific immune system is one of the two main immunity strategies in vertebrates (the other being the adaptive immune system). The innate immune system is an alternate defense strategy and is the dominant immune system response found in plants, fungi, prokaryotes, and invertebrates (see § Beyond vertebrates). The major functions of the innate immune system are to: recruit immune cells to infection sites by producing chemical factors, including chemical mediators called cytokines activate the complement cascade to identify bacteria, activate cells, and promote clearance of antibody complexes or dead 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

Key Points

  • The complement system is a key mechanism of innate immunity that utilize proteins to identify and eliminate pathogens, while also maintaining homeostasis by clearing dead and damaged cells.
  • Viral components are recognized by different receptors: Toll-like receptors are located in the endosomal membrane and recognize double-stranded RNA (dsRNA), MDA5 and RIG-I receptors are located in the cytoplasm and recognize long dsRNA and phosphate-containing dsRNA respectively.
  • Inflammation is one of the first responses of the immune system to infection or irritation.
  • Phagocytosis of the hosts' own cells is common as part of regular tissue development and maintenance.
  • Neutrophils, along with eosinophils and basophils, are known as granulocytes due to the presence of granules in their cytoplasm, or as polymorphonuclear cells (PMNs) due to their distinctive lobed nuclei.
  • Macrophages are the most efficient phagocytes and can phagocytose substantial numbers of bacteria or other cells or microbes.

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