immune system

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

Observe the human immune response phagocytosis

Labeled illustration of a macrophage showing its internal organelles and the process of engulfing and digesting bacteria, with bacteria drawn outside and inside the cell as it undergoes phagocytosis.

Diagram of the activities of T and B cells

Diagram of the human lymphatic system highlighting lymphatic vessels and major lymphoid organs, including lymph nodes, adenoids, tonsils, thymus gland, spleen, appendix, bone marrow, and ducts.

At a Glance

immune system summary

immune system

physiology

Also known as: immunological system

Written by

Samuel Scott PerdueAll

Fact-checked by

Britannica Editors

Last updated

Nov. 12, 2026 •History

immune system, the complex group of defense responses found in humans and other advanced vertebrates that helps repel disease-causing organisms (pathogens). Immunity from disease is actually conferred by two cooperative defense systems, called nonspecific, innate immunity and specific, acquired (or adaptive) immunity. Nonspecific protective mechanisms repel all microorganisms equally, while the specific immune responses are tailored to particular types of invaders. Both systems work together to thwart organisms from entering and proliferating within the body....

Discover the world with Britannica

Subscribe to Continue Reading

Free Trial | No Ads | Exclusive Content

Explore Subscriptions

immune system

Subscribe to Continue Reading

Thank you for signing up!