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Anatomy & Physiology/Lesson 6 of 7

Immunology

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The immune system protects the body from pathogens, foreign substances, and abnormal cells.

Functions of the Immune System

  • Prevent infection
  • Eliminate pathogens
  • Remove damaged cells
  • Destroy abnormal (cancerous) cells
  • Create immune memory

Innate vs Adaptive Immunity

  • Innate Immunity
    • Rapid response
    • Nonspecific
    • No memory
    • Present at birth
    • Examples: phagocytes, inflammation
  • Adaptive Immunity
    • Slower first response
    • Antigen-specific
    • Long-term memory
    • Develops after exposure
    • Examples: B cells, T cells, antibodies

Innate Immunity

First Line of Defense

  • Skin
  • Mucous membranes
  • Stomach acid
  • Tears
  • Saliva

These physical and chemical barriers prevent pathogens from entering the body.

Inflammation

Signs:

  • Redness
  • Heat
  • Swelling
  • Pain

Purpose:

  • Recruit immune cells
  • Remove pathogens
  • Begin healing

Phagocytosis

Major phagocytic cells:

  • Neutrophils
  • Macrophages
  • Dendritic cells

Simple pathway:

Pathogen → Phagocyte → Engulfment → Digestion → Removal

White Blood Cells (Leukocytes)

Leukocytes are the cellular defenders of the immune system. Major types include:

  • Neutrophils: Rapid first responders; highly phagocytic
  • Lymphocytes: B cells, T cells, and NK cells
  • Monocytes: Circulate in blood and become macrophages in tissues
  • Eosinophils: Fight parasites and participate in allergic responses
  • Basophils: Release histamine during allergic reactions

Adaptive Immunity

B Cells

B cells originate in the bone marrow. Their functions include:

  • Recognize antigen
  • Become plasma cells
  • Produce antibodies
  • Become memory B cells

Simple pathway:

Antigen → B-cell activation → Plasma cells + Memory B cells

Antibodies

Antibodies are proteins produced by plasma cells. Their functions include:

  • Bind specific antigens
  • Neutralize pathogens or toxins
  • Tag pathogens for destruction
  • Help activate other immune defense mechanisms

Memory Cells

Memory cells produce a faster and stronger response during future infections.

T Cells

T cells mature in the thymus. Major types include:

  • Helper T cells: Activate B cells, activate macrophages, release cytokines, and coordinate the immune response
  • Cytotoxic T cells: Destroy virus-infected cells, cancer cells, and other abnormal cells

Humoral vs Cell-Mediated Immunity

  • Humoral immunity: Mediated by B cells and antibodies; targets pathogens outside cells
  • Cell-mediated immunity: Mediated by T cells, especially cytotoxic T cells; targets infected cells

Antigen

An antigen is a substance or molecular structure that can be specifically recognized by components of the adaptive immune system, such as antibodies or antigen receptors on B and T cells.

Antigen-Presenting Cells (APCs)

APCs capture and present antigens to T cells. Examples include:

  • Dendritic cells
  • Macrophages
  • B cells

Cytokines

Cytokines are chemical messengers released by immune cells that coordinate the immune response. Key examples:

  • IL-1 — promotes inflammation and fever.
  • IL-2 — key T-cell growth factor; stimulates proliferation of T cells.
  • IL-6 — drives acute-phase response and fever.
  • TNF-α — promotes inflammation and can trigger septic shock at high levels.
  • Interferons (IFN-α/β/γ) — antiviral signaling and macrophage activation.

Clinical Applications

  • Vaccination: Stimulates adaptive immunity and memory formation without causing disease
  • Allergy: An exaggerated immune response to harmless environmental antigens
  • Autoimmune disease: The immune system mistakenly attacks the body's own tissues
  • Immunodeficiency: Reduced immune function increases susceptibility to infection
  • HIV/AIDS: HIV infects and depletes CD4 helper T cells, weakening adaptive immunity
  • Cancer immunosurveillance: The immune system recognizes and destroys abnormal cancer cells

Key Points

  • The immune system prevents infection, eliminates pathogens, removes damaged cells, and creates immune memory
  • Innate immunity is rapid, nonspecific, and present at birth; adaptive immunity is antigen-specific and creates memory
  • The first line of defense includes skin, mucous membranes, stomach acid, tears, and saliva
  • Inflammation recruits immune cells, removes pathogens, and begins healing
  • Phagocytosis is performed by neutrophils, macrophages, and dendritic cells
  • B cells produce antibodies and memory cells; T cells mature in the thymus and include helper and cytotoxic types
  • Cytokines are chemical messengers that coordinate immune responses
  • Key points include vaccination, allergy, autoimmune disease, immunodeficiency, HIV/AIDS, and cancer immunosurveillance
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