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Innate Immunity Part B

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MHC 1 molecule

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Complement Proteins

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Innate Immunity

Innate immunity is the non-specific, in-born immune system. This system is an important subsystem of the larger, overall immune system that is comprised of cells and mechanisms designed to defend the host from infections. The cells in this innate system respond to pathogens in a non-specific, or generic way. They act as a sort of “shotgun blast” first response at the first sign of infection. While the innate immune system is at work, the acquired immune system builds specialized antibodies that will be more effective against the specific pathogen, but useless against others. In short, innate immunity is a short term defense meant to hold off infection long enough for the body to respond properly.

The major functions of innate immunity include:

  • Acting as a chemical or physical barrier to infections
  • Activating the adaptive immune system through antigen presentation
  • Removal and identification of foreign bodies in blood, lymph, organs, and tissues
  • Activating the complement cascade in identifying bacteria, activating cells, promoting clearance of dead cells or antibody complexes
  • Mobilizing immune cells to the site of an infection, by producing chemical factors and specialized chemical mediators known as cytokines

Anatomical Barriers

Anatomical barriers are both physical and chemical barriers to defend the host from foreign bodies. The physical anatomical barriers are the first line of defense against invading infections or viruses. Below are some of the ways the body utilizes anatomical barriers:

  • Skin – Flushing, organic acids, desquamation (shedding or peeling dead skin cells), and sweat
  • Eyes – Tears
  • Nasopharynx (Nasal) – Lysozyme, Saliva, Mucus
  • Lungs and Respiratory Airways – Defensins, surfactant, mucociliary elevator
  • Gastrointestinal Tract – Flushing, gut flora, defensins, gastric acid, peristalsis, bile acids, thiocyanate, digestive enzyme


One of the first responses of the immune system to irritation or infection is inflammation. This response is stimulated by various chemical factors that are released by injured cells to serve as a physical barrier to prevent an infection from spreading. These chemical factors will also promote healing of the damaged tissues after the pathogens have passed.

Inflammation can be characterized by the following identifiable symptoms:

  • Redness
  • Warm feeling, heat, or a systemic fever
  • Swelling of the affected area/tissues
  • Increased mucus production, can cause runny nose or a cough
  • General pain – can be around the affected area, in joints, a sore throat, or even just body aches

Overall, inflammation is a sign that your innate immunity is doing its job in preventing the spread of an infection. Most symptoms of inflammation subside quickly and healing begins.

Complement System

innate-immunityThe complement system is a biochemical cascade within the immune system that helps, or complements, the antibodies with clearing pathogens or marking them for eradication by other cells. This system is comprised of several plasma proteins that are synthesized in the liver by hepatocytes. These proteins will work together to accomplish:

  • A trigger to recruit inflammatory cells
  • “Tagging” pathogens for eradication by other cells through opsonizing (or coating) the pathogen’s surface
  • Forming holes in the membrane of the plasma of a pathogen, causing the pathogen to die
  • Ridding the host of any neutralized antigen-antibody complex

Cells of the In-Born Immunity System

The innate immune system also employs a number of different types of cells, each of which has a specific job when infection strikes.

  1. Mast Cells – For wound healing and defending against pathogens. Also associated with allergies and anaphylaxis (allergic reaction). Mast cells release granules that are rich in histamines and heparins. Histamines dilate the blood vessels and cause general inflammation.
  2. Phagocytes – The literal translation of phagocyte is ‘eating cell.’ These cells completely engulf pathogens or foreign particles. Once engulfed, a chemical reaction occurs that uses acids to kill and digest the organism or particle.
  3. MacrophageMacrophages – The literal translation of macrophages is ‘large eaters.’ These cells move freely outside of designated specific areas (some cells are contained to muscle tissue, bone tissue, etc.) to pursue pathogens. Typically, a macrophage responds to a marker on a cell that triggers it to engulf and eradicate the bacteria through the creation of a respiratory burst. Certain pathogens will also stimulate macrophage to produce chemokines to summon other types of cells to the site of an infection.
  4. Neutrophils – This cell contains a particular type of granule that is comprised of a variety of toxic substances that will kill or prevent the growth of bacteria and fungus. Neutrophils also utilize a respiratory burst to eradicate cells.
  5. Dendritic Cells – These cells are typically present in tissues that make contact with the external environment – such as the skin.
  6. Natural Killer Cells – These are cells that are activated through the “missing self” condition where the cells react and respond to similarly marked cells. These cells typically respond to tumor cells or virally-infected cells.

Innate Immunity Overview

Innate immunity is a system in which your body automatically tries to fight off infection, viruses, tumors, and more. Unlike the adaptive immune system, your innate immune system does not grow stronger as it defeats various infections and viruses. It relies on a system of cells that will either destroy a foreign body or infection, or mark it to be destroyed. The cells also form a chemical reaction that will mobilize more specialized cells to the fight. Some innate immunity cells also create physical or chemical barriers to prevent infection or disease from even entering the body. Inflammation usually the first outward sign of infection, but it is also a sign that your innate immunity is doing its job!

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