Immune System Overview

Immune System Overview

• The immune system is a complex network of tissues, cells and molecules.
• It defends the body against infections, foreign substances and abnormal cells (like cancer).

Learning Objectives

• Define the immune system
• Identify and list the cells of the immune system
• Know the function of B and T lymphocytes

Immune System Tissues

• Primary Lymphoid Organs

  • Bone Marrow: Site of hematopoiesis (blood cell production); produces B cells (mature in bone marrow) and immature T cells.
  • Thymus: Site where T cells mature and undergo selection to become functional.

• Secondary Lymphoid Organs

  • Lymph Nodes: Filter lymphatic fluid, house immune cells to detect and respond to antigens.
  • Spleen: Filters blood, removes pathogens, old red blood cells and debris; contains white pulp (immune function) and red pulp cells.
  • Mucosa-Associated Lymphoid Tissue (MALT): Provides localized immune defense at mucosal surfaces (e.g., tonsils, Peyer's patches, appendix).
  • Skin-Associated Lymphoid Tissue (SALT): Immune components in the skin protect against pathogens breaching the skin barrier.

Cells of the Immune System

• Innate Immune Cells (Immediate, Non-specific Response):

  • Neutrophils: Rapid responders that engulf and destroy bacteria and fungi.
  • Macrophages: Large phagocytes engulfing pathogens and dead cells, presenting antigens to T cells.
  • Dendritic Cells: Professional antigen-presenting cells linking innate and adaptive systems.
  • Natural Killer (NK) Cells: Destroy virus-infected and abnormal cells through apoptosis.
  • Mast Cells: Release histamine and other chemicals during allergic responses and inflammation.
  • Eosinophils: Involved in combating parasitic infections and allergic reactions.
  • Basophils: Release inflammatory mediators and play a role in allergies.

• Adaptive Immune Cells (Specific, Slower Response, with Memory):

  • B Lymphocytes (B Cells): Produce antibodies to neutralize pathogens; differentiate into plasma cells (antibody-producing cells) and memory B cells.
  • T Lymphocytes (T Cells):
    • Helper T cells (CD4+): Activate other immune cells (including B cells and cytotoxic T cells).
    • Cytotoxic T cells (CD8+): Kill virus-infected and cancerous cells.
    • Regulatory T cells: Suppress immune responses to maintain tolerance and prevent autoimmunity.
    • Memory T cells: Long-lived cells providing faster responses upon re-exposure to antigens.

• Other Immune Cells:

  • Monocytes: Precursors to macrophages and dendritic cells, circulating in the blood.
  • Platelets: Involved in blood clotting and release immune mediators during inflammation.

• Antigen-Presenting Cells (APCs): Specialized cells processing and presenting antigens to T cells, initiating adaptive immune responses. Examples include dendritic cells and macrophages.

T Lymphocytes

• 80% of circulating lymphocytes
• Cytotoxic T cells (CD8): Attack foreign cells or body cells infected by viruses (cell-mediated immunity).
• Regulatory T cells (CD4): Helper T cells and suppressor T cells control activation and activity of B cells.

CD Nomenclature

• The CD nomenclature is a standardized system to classify and identify molecules on the surface of immune cells, including lymphocytes.
• CD molecules are used to classify leukocytes into functionally distinct groups (e.g., helper T cells).
• CD molecules are often involved in leukocyte functions.

B Lymphocytes

• B lymphocytes are formed in the bone marrow and develop there. They interact with antigenic epitopes using immunoglobulin receptors. Development into plasma cells secrete specific antibodies.
• Immature B cells express IgM receptors. Mature B cells express IgM and IgD molecules as antigen receptors.
• B cells have receptors for the Fc portion of IgG and a receptor for the C3 component of the complement.

Natural Killer (NK) Cells

• 5-10% of circulating lymphocytes
• Attack foreign cells, normal cells infected with viruses, and cancer cells.
• Known as "immunologic surveillance."

Antibody Structure

• Antibodies are composed of two identical heavy chains and two identical light chains. Specific antigen-binding sites on antibodies recognize pathogens, binding to them and marking them for destruction by complement activation or phagocytes.

Other details in the slides:

• Th1 & Th2 Subsets: Different types of helper T cells (Th1 and Th2) produce different cytokines.
• CD8 Cytotoxic T-lymphocytes: Constitute 35% of peripheral T cells; Recognize antigens on target cells (infected cells or tumor cells) in association with MHC proteins. Activate and kill virus-infected cells or tumor cells.