Overview of the Immune System

Key Functions

• Protects against microorganisms, removes toxins, destroys tumor cells.
• Generates immune responses to identify threats, attack, eliminate pathogens, and remember offenders.

Two Main Branches

1. Innate Immune Response

   • Non-specific, rapid (minutes to hours), no memory.
   • Includes physical barriers (skin, gut epithelium) and chemical barriers (low pH in the stomach).

2. Adaptive Immune Response

   • Highly specific for each invader, diverse, and can recognize a vast number of antigens.
   • Slower initiation (weeks to peak), but develops immunologic memory.

Important Concepts

• Immunologic Memory: Adaptive immune response generates a stronger, faster response upon re-exposure to the same pathogen.
• Hematopoiesis: Formation of blood cells (white and red blood cells, platelets) in the bone marrow.

Components of the Immune System

White Blood Cells (Leukocytes)

• Hematopoietic Stem Cells: Differentiate into various cell types.
  • Myeloid Progenitor Cells: Develop into myeloid cells (neutrophils, eosinophils, basophils, etc.).
  • Lymphoid Progenitor Cells: Develop into lymphocytes (B cells, T cells, natural killer cells).

Myeloid Cells (Innate Immunity)

• Neutrophils: Most abundant, phagocytosis of pathogens, oxidative burst.
• Eosinophils & Basophils: Contain granules with inflammatory molecules; eosinophils fight large parasites, involved in allergic reactions.
• Monocytes and Macrophages: Engulf pathogens, present antigens, release cytokines.
• Dendritic Cells: Key antigen-presenting cells, connect innate and adaptive immunity.

Lymphoid Cells

• B Cells: Have receptors to bind specific antigens, can become plasma cells to secrete antibodies (humoral immunity).
• T Cells: Involved in cell-mediated immunity, specific antigen recognition.
  • CD4 T Cells (Helper T Cells): Secrete cytokines, aid macrophages and B cells.
  • CD8 T Cells (Cytotoxic T Cells): Kill cells presenting specific antigens.
• Natural Killer Cells: Part of the innate immune system, kill infected or cancerous cells.

Immune Response Process

Initial Invasion

1. Pathogen Breaches Barriers: Breathed in, bypasses barriers like nose hairs and epithelium.
2. Innate Response Activation: Macrophages ingest bacteria, release cytokines, attract eosinophils, basophils, and mast cells.

Bridging Innate and Adaptive Responses

• Dendritic Cells: Ingest pathogens and move to lymph nodes, present antigens to naive T cells.

Adaptive Response Activation

• T and B Cell Activation: T cells differentiate, release cytokines, B cells become plasma cells to produce antibodies.
• Antibodies: Tag pathogens for destruction.

Memory Formation

• Memory B and T Cells: Form for long-term immunity.

Summary

• The immune system is divided into innate (immediate, non-specific) and adaptive (specific, memory-based) responses.
• Key players include various white blood cells that perform distinct roles in immune defense.