immune system

Table of Contents

Introduction & Top Questions
Mechanisms of the immune system
- Nonspecific, innate immunity
  - External barriers to infection
    - Skin
    - Mucous membranes
  - Chemical barriers to infection
    - Chemicals with incidental protective effects
    - Antimicrobial proteins
      Complement
      Interferons
    - Proteins from naturally occurring bacteria
  - Cellular defenses
    - Scavenger cells
      Granulocytes
      Macrophages
    - Natural killer (NK) cells
  - Nonspecific responses to infection
    - Acute-phase response
    - Inflammatory response
- Specific, acquired immunity
  - The nature of lymphocytes
    - General characteristics
      Location in the lymphatic system
      T and B cells
    - Ability to recognize foreign molecules
      Receptor molecules
      Antigens
    - Diversity of lymphocytes
  - B-cell antigen receptors and antibodies
    - Basic structure of the immunoglobulin molecule
    - Classes of immunoglobulins
      IgG
      IgM
      IgA
      IgD
      IgE
    - Normal production of antibody
  - T-cell antigen receptors
    - Structure of the T-cell receptor
    - Function of the T-cell receptor
  - Life cycle of T and B lymphocytes
    - T cells
    - B cells
  - Activation of T and B lymphocytes
    - Activation of T cells
      Helper-T-cell activation
      Results of helper-T-cell activation
    - Activation of B cells
  - Antibody-mediated immune mechanisms
    - Protective attachment to antigens
    - Activation of the complement system
    - Activation of killer cells
    - Other antibody-mediated mechanisms
    - Transfer of antibodies from mother to offspring
  - Cell-mediated immune mechanisms
  - Immunity against cancer
  - Prophylactic immunization
    - Passive immunization
    - Active immunization
Evolution of the immune system
- The development of immunity in major animal groups
  - Immune capacity among invertebrates
  - Immune capacity among vertebrates
- Genetic origins of the immune system

References & Edit History Quick Facts & Related Topics

Images, Videos & Interactives

Immune stimulation by activated helper T cells

Human body systems interactive

Clonal selection of a B cell

Observe the human immune response phagocytosis

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Macrophage structure

Macrophage attacking a cancer cell

Diagram of the activities of T and B cells

Diagram of the human lymphatic system

For Students

immune system summary

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immune system: Facts & Related Content

Written and fact-checked by The Information Architects of Encyclopaedia Britannica

Last Updated: Jun 03, 2025

News •

Bladder cancer treatment reprograms bone marrow to boost immune system's cancer-fighting ability • May 30, 2025, 12:54 AM ET (Medical Xpress)

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Facts

Also Known As	immunological system

Photos and Videos

Immune stimulation by activated helper T cells

Clonal selection of a B cell

Macrophage structure

Macrophage attacking a cancer cell

Diagram of the activities of T and B cells

Diagram of the human lymphatic system

Antigens, antibodies, and lymphocytes

Antibody structure

Domains of an antibody

Observe the human immune response phagocytosis

What is the field of immunogenomics?

immune-mediated diseases

Why do people have allergies in the spring?

Can autoimmune diseases be treated more effectively?

See All Videos, Images and Interactives →

Topics

random migration scavenger cell acquired immunological tolerance pyrogen immunity immunological memory chemotaxis antibody white blood cell lymph muscle nervous system circulatory system respiratory system integument endocrine system mucous membrane gland human ear physiology

Dig Deeper: More Articles That Discuss This Topic

Key People

Bruce A. Beutler.

Bruce A. Beutler

American immunologist

Ralph M. Steinman.

Ralph M. Steinman

Canadian immunologist and cell biologist

Jules Hoffmann

Jules Hoffmann

French immunologist

James P. Allison

American immunologist

Tonegawa Susumu

Japanese biologist

Niels K. Jerne

Danish immunologist

Bordet, Jules

Jules Bordet

Belgian bacteriologist

Quiz

3d illustration human heart. Adult Anatomy Aorta Black Blood Vessel Cardiovascular System Coronary Artery Coronary Sinus Front View Glowing Human Artery Human Heart Human Internal Organ Medical X-ray Myocardium

Human Organs

List

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Demystified

Macro Aedes aegypti Mosquito. Close up a Mosquito sucking human blood,

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