Innate Immune System- An Introduction


The innate immune system is one of the two main immunity strategies in vertebrates, along with the adaptive immune system. The innate immune system is also the dominant immune system response found in plants, fungi, insects, and primitive multicellular organisms.

The innate immune system provides a preconfigured response to broad groups of situations and stimuli, such as pathogens (disease-causing organisms) and tissue damage. The innate immune system responds in the same general way to every pathogen it encounters, regardless of its specificity or identity. The innate immune system is also called the nonspecific immune system because of this feature.

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
  • identify and remove foreign substances present in organs, tissues, blood and lymph, by specialized white blood cells
  • activate the adaptive immune system through antigen presentation
  • act as a physical and chemical barrier to infectious agents; via physical measures such as skin and chemical measures such as clotting factors in blood, which are released following a contusion or other injury that breaks through the first-line physical barrier

The innate immune system consists of several components, such as:

  • anatomical barriers: physical, chemical and biological barriers that prevent or limit the entry of pathogens into the body, such as skin, mucous membranes, gut flora, and tears
  • cellular components: white blood cells that perform various functions, such as phagocytosis (engulfing and destroying microbes), cytotoxicity (killing infected cells), inflammation (increasing blood flow and permeability), and antigen presentation (displaying fragments of pathogens to activate adaptive immunity), such as neutrophils, macrophages, dendritic cells, natural killer cells, and mast cells
  • molecular components: soluble proteins and molecules that have various roles in recognizing and eliminating pathogens, such as complement proteins, cytokines, chemokines, interferons, defensins, and acute-phase proteins

The innate immune system is essential for the survival of vertebrates, as it provides immediate protection against infections and tissue damage. It also prepares the body for a more specific and effective response by the adaptive immune system. However, the innate immune system has some limitations, such as:

  • lack of specificity: the innate immune system cannot distinguish between different types of pathogens or remember previous encounters with them
  • lack of diversity: the innate immune system has a limited number of receptors and molecules that can recognize pathogens
  • lack of regulation: the innate immune system can cause excessive inflammation or tissue damage if not controlled properly

In this article, we will discuss some of the main components of the innate immune system in more detail: complement system, cytokines, natural killer cells, neutrophils, and macrophages. We will also explain how they interact with each other and with the adaptive immune system to provide a comprehensive defense against pathogens.