Describe the structure and function of the human immune system.

The human immune system is a complex network of cells, tissues, and organs that work together to defend the body against pathogens (e.g., bacteria, viruses, fungi) and foreign substances. It is broadly divided into two main components: innate immunity (non-specific) and adaptive immunity (specific).

Structure of the Immune System

1. Primary Lymphoid Organs (where immune cells develop):

   • Bone marrow: Produces all blood cells, including lymphocytes (B-cells and T-cells).
   • Thymus: Maturation site for T-cells.

2. Secondary Lymphoid Organs (where immune responses are initiated):

   • Lymph nodes: Filter lymph and trap pathogens.
   • Spleen: Filters blood and removes pathogens.
   • Tonsils and adenoids: Trap pathogens entering via the respiratory tract.
   • Mucosa-associated lymphoid tissue (MALT): Found in mucous membranes (e.g., gut, respiratory tract).

3. Cells of the Immune System:

   • Innate Immunity:
     • Phagocytes (neutrophils, macrophages): Engulf and destroy pathogens.
     • Natural Killer (NK) cells: Destroy virus-infected and cancerous cells.
     • Dendritic cells: Present antigens to activate T-cells.
   • Adaptive Immunity:
     • B-cells: Produce antibodies (humoral immunity).
     • T-cells: Include helper T-cells (activate B-cells and macrophages), cytotoxic T-cells (kill infected cells), and regulatory T-cells (suppress immune responses).

Functions of the Immune System

1. Innate Immunity:
   • Provides immediate, non-specific defense.
   • Recognizes pathogens via pattern recognition receptors (PRRs).
   • Triggers inflammation to isolate and eliminate pathogens.

2. Adaptive Immunity:
   • Specificity: Recognizes and targets specific pathogens.
   • Memory: Remembers past infections for faster responses (vaccination works on this principle).
   • Diversity: Produces a wide range of antibodies and T-cell receptors.

Key Processes

• Antigen Presentation: Macrophages and dendritic cells present antigens to T-cells.
• Antibody Production: B-cells produce antibodies that neutralize pathogens.
• Cell-Mediated Immunity: Cytotoxic T-cells destroy infected host cells.
• Inflammation: A response to tissue damage or infection, involving vasodilation, increased permeability, and recruitment of immune cells.

The immune system maintains homeostasis by balancing immune responses to prevent autoimmunity (attacking self-cells) and immunodeficiency (weakened immunity).