Homeostasis (from the Greek words for “same” and “steady”) refers to the state of balance and stability that allows the body to maintain its equilibrium and overall health. Homeostasis includes processes involved with maintaining body temperature, blood glucose levels, arterial blood pressure.
Immune homeostasis considers this state of equilibrium in relation to the immune system.
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Negative Feedback Loops and Maintaining Homeostasis
Maintaining homeostasis involves negative feedback loops.
Negative feedback loop – Reaction that results in decreased function, often occurring in response to a stimulus (“negative” here refers to “opposite”, not necessarily “bad”).
A SENSOR detects the STIMULUS (a change). The CONTROL CENTER processes the information detected by the SENSOR and directs the EFFECTOR to induce a physiological change.
Immune homeostasis involves countless dynamic mechanisms that constantly have to contend with:
- Cell proliferation and cell death occurring in different locations and lineages
- Responses to pathogens, drugs, and cancers
- Chronic conditions
- Stressors from microorganisms
- Environmental challenges
Immune homeostasis also depends on homeostasis of specific immune cells, including:
- Dendritic cells
- T-cell lymphocytes
- Natural killer cells
Lines of Defense in the Immune System
The immune system features several lines of defense to maintain immune homeostasis.
First line of defense – Physical barriers between the inside of the body and the outside environment
- Stomach lining and acids
- Mucus membranes
Second line of defense – Innate immunities activated when immune system recognizes molecules on invading substances instead of cells of the body; comprises types of white blood cells and chemical messengers, including:
Third line of defense – Specific resistance (acquired immunities), resulting in the creation of antibodies that attack invading microbes and prevent future infections
The negative feedback loop involved in homeostasis applies to immune homeostasis:
Stimulus: Pathogenic microbes enter the body and challenge the immune system. Viruses may kill cells or disrupt cellular functions. Bacteria may act similarly, produce toxins, or multiply and crowd out healthy tissue.
Sensor: Immune cells detect the invading microbe.
Control center: Immune system identifies the type of microbe involved (virus, bacteria, parasite, etc.) and deploys specific measures to combat the infection.
Effector: White blood cells, antibodies, and other immune cells attempt to remove the pathogen. Many symptoms of illness (fever, headaches, physical fatigue) actually come from the immune response.
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