Immune System（免疫系）とは？

The immune system is a network of cells, proteins, and organs that protects the body from pathogens such as bacteria, viruses, fungi, and parasites. It plays a critical role in maintaining health by identifying and eliminating harmful invaders and abnormal cells.

When functioning properly, the immune system operates unnoticed. If impaired, it may fail to defend against infections or mistakenly attack healthy cells, leading to disease. Its capacity to recognize and respond to previously encountered pathogens underpins immunity acquired through infection or vaccination.

Functions of the Immune System

The immune system serves three primary functions:

1. Defending Against Pathogens: Identifies and removes harmful microorganisms and other external threats 

2. Neutralizing Harmful Substances: Detects and neutralizes toxins or allergens from the environment 

3. Eliminating Diseased Cells: Identifies and removes abnormal or mutated cells, such as cancer cells. 

Activation of the Immune System

The immune system is activated by antigens, which are molecules that the body recognizes as foreign. Antigens, such as proteins on the surface of bacteria and viruses, bind to specific receptors on immune cells, triggering a series of responses. 

Following initial exposure, the immune system retains information about the pathogen. This memory enables a faster and more effective response to future encounters. This mechanism is the basis for long-term immunity provided by vaccines and prior infections. 

Autoimmune responses occur when the immune system mistakenly attacks healthy cells, perceiving them as foreign. 

Innate and Adaptive Immune Systems

1. Innate Immune System

This is the first line of defense, offering a rapid, non-specific response to pathogens. It relies on barriers like the skin and immune cells such as phagocytes and natural killer cells to contain and eliminate threats. 

2. Adaptive Immune System

The adaptive system provides a targeted response through antibodies produced by B-cells. It also generates memory cells, enabling rapid recognition of previously encountered pathogens. Unlike the innate system, the adaptive immune system learns and adapts over time, which is needed for combating evolving pathogens. 

Components of the Immune System

• White Blood Cells: Phagocytes ingest harmful particles, while lymphocytes (T-cells and B-cells) coordinate specific immune responses. 

• Antibodies: Proteins produced by B-cells that bind to and neutralize antigens. 

• Organs and Tissues: Bone marrow, thymus, lymph nodes, and spleen support immune cell production and activation. 

• Signaling Molecules: Cytokines and chemokines regulate communication and direct immune activity. 

Challenges and Dysregulation

The immune system faces challenges in responding to pathogens and maintaining balance: 

• Weakened Immunity: Chronic illness, malnutrition, or immunosuppressive therapies can impair immune responses. 

• Overactive Immunity: Allergies and autoimmune disorders result from inappropriate or excessive immune activity. 

• Pathogen Evolution: Some pathogens, such as influenza viruses, mutate rapidly, complicating immune recognition and response. 

Immune System Applications in Medicine

• Immunotherapy: Cancer treatments such as monoclonal antibodies enhance immune system activity. 

• Vaccines: Stimulate the adaptive immune system to provide long-term protection against specific pathogens. 

• Diagnostics: Antibody-based tests like ELISA leverage the immune system’s specificity to detect diseases. 

Related: Antibodies: Applications as Tools

References:

1. Institute for Quality and Efficiency in Health Care (IQWiG). (2023, June 6). In brief: How does the immune system work? Nih.gov; Institute for Quality and Efficiency in Health Care (IQWiG). https://www.ncbi.nlm.nih.gov/books/NBK279364/