Introduction

The Innate immune system is the body's first barrier of defence to infection. It relies on an older, more generic, and faster acting set of tools than the adaptive system. While the adaptive system is essential for a specific response to infection, it is ultimately the innate system that conquers foreign attackers through means of phagocytosis.

• Non-specific protective mechanisms include such innate factors as:
  • Physical barriers
    • Skin
    • Ciliated mucous membranes
    • Commensal organisms
  • Humoral factors
    • Lysozyme
    • Complement
    • Interferons
  • Cellular mechanisms
    • Phagocytosis
  • Factors which regulate species specificity
    • Membrane receptors for pathogens
    • Nutritional requirements
    • Temperature
    • pH
• Mechanisms of innate immunity are always present and generally unchanging
• Adaptive immunity is acquired only on contact with the infectious agent (antigen) and therefore does not function before first contact with the antigen

Actions of the Innate Immune System

Recognition of Microorganisms

Phagocytosis

Tools of Innate Immunity

Innate Immunity Barriers

Humoral Factors of Innate Immune System

Innate Immunity Cellular Responses

Innate Immunity to Viruses

Innate response to dsRNA - B. Catchpole, RVC 2008

Because viruses invade host cells to take over a host's cellular machinery, the innate system has a more difficult time detecting viruses as foreign agents. However, there is a give-away element of the viral attack that the innate system can recognize: the double-stranded RNA (dsRNA) produced by a virus in its replication phase. Because mammalian cells only ever produce single-stranded RNA, the presence of dsRNA signals a foreign intruder. dsRNA can be detected by TLR-3R on the cell surface or intracellularly by the presence of dsRNA-dependent protein kinase.

The innate response to viral attack also depends on the presence of Type-1 Interferons, which are produced by all cells on recognition of a viral attack. Interferons serve to increase degradation of mRNA, inhibit protein synthesis, and increase the effectiveness of the adaptive response by increasing antigen presentation to antibody.

Lastly, the final line of defense for the innate response to viruses lies in the actions of Natural Killer (NK) cells. These warriors monitor the production of MHC (Major Histocompatibility Complex) on the surface of cells, which is produced as part of the adaptive response. A cell whose cellular machinery is compromised by viral infection will experience a drop in the amount of MHC it produces. When a cell's MHC production drops, NK cells are triggered to phagocytose these cells. As such, this is a non-specific targeting based simply on the ability of a cell to function normally, which also lends them to playing a role in targeting malignant cells. NK cells are incapable of directly targeting viral infection.

Innate Immunity to Bacteria

Bacterial responses - B. Catchpole, RVC 2008

The innate response to bacterial infection lies in its first-response role of detection of a foreign organism. By using the above described tools of Pattern-Recognition Receptors (PRRs), the innate response flags up problems while the adaptive response gets itself organized. Once a foreign organism is detected, the innate system responds by engaging in cell warfare via phagocytosis and triggering the inflammatory response. The release of inflammatory cytokines will cause an increase in vasodilation, vascular permeability and an influx of white blood cells. Neutrophils take on their primary role as phagocytes in this phase. In addition, systemic effects of inflammatory cytokines will sustain a rise in core temperature (fever), the release of acute phase proteins from the liver, and bone marrow mobilization as the need for white blood cells production is increased. Acute phase proteins will bind to bacterial cell walls, enhancing neutrophil, macrophage, and complement-initiated phagocytosis.

Interplay of Innate and Adaptive Immunity

Innate Immunity Flashcards

Links

Websites

• http://www.cellsalive.com

References