The Immune and Lymphoid Systems

The Immune System

• The immune system is the body's defense mechanism against pathogens.
• It has different layers of organs responsible for preventing and fighting infection.
• It functions by:
  • Keeping invaders out.
  • Destroying invaders.
  • Limiting harm caused by invaders.
  • Healing damage.
  • Adapting to new threats.
• It works by:
  • Distinguishing "self" from "non-self" cells.
  • Mobilizing to kill harmful germs.
  • Ending attacks after threats are gone.
  • Developing antibodies against germs after exposure.
  • Sending antibodies to destroy future invaders.

The Lymphoid System

• It's a network of vessels, nodes, and ducts that circulate lymph throughout the body.
• It's part of the immune system and is essential for immune function.
• It includes central and peripheral lymphoid organs.

Central Lymphoid Organs

• These include the bone marrow and thymus.
• The bone marrow functions as the primary site for hematopoiesis in infants and children.
• It produces red blood cells, white blood cells, and platelets.
• The thymus is important for T-cell maturation and development.
• It becomes more dominant with age and is gradually replaced by yellow marrow.

Peripheral Lymphoid Organs

• These are also known as secondary lymphoid organs.
• They include:
  • Tonsils: first line of defense against bacteria and viruses, located in the back of the throat.
  • Adenoids: Located behind the nasal cavity, active during childhood.
  • Lymph nodes: Bean-shaped glands that filter lymph, clear out damaged cells and cancer cells, and store lymphocytes.
  • Mucosa-associated Lymphoid Tissue (MALT): Found in the lining of the digestive, respiratory, and urinary systems.
  • Spleen: Filters blood and stores lymphocytes, removes old red blood cells.

Normal Immune Response

• It maintains an immune balance, removing damaged cells and preventing infection.
• It's a balanced response of circulating and resident lymphocytes.
• It involves surveillance, which enables the body to recognize and remember pathogens.

Types of Immunity

• Acquired Immunity: Long-term response with antibody development.
  • Humoral Immunity: Antibody-mediated, involves recognition of antigens by B lymphocytes.
    • B lymphocytes differentiate into plasma cells and memory cells.
    • Plasma cells secrete immunoglobulins, which target specific antigens.
  • Cell-Mediated Immunity: Involves activation of phagocytes and cytotoxic T-lymphocytes.
    • T lymphocytes recognize specific major histocompatibility complexes (MHC) and elicit an immune response.
    • CD markers are protein markers on T-cells that help identify their function.

Immunoglobulins (Ig)

• Also known as antibodies, are glycoproteins produced by plasma cells.
• They recognize and bind to specific antigens, aiding their destruction.
• There are five classes of immunoglobulins:
  • IgA: Found in body secretions, protects body surfaces.
  • IgD: Present on lymphocytes.
  • IgE: Involved in allergic reactions, binds to mast cells.
  • IgG: Most abundant immunoglobulin, crosses placenta, involved in secondary immune responses.
  • IgM: First immunoglobulin produced during infection, larger than other classes.

Other Immune System Participants

• Natural Killer Cells (NK Cells): Present at birth, do not require prior sensitization, and are not T- or B-lymphocytes.
• Cytokines: Soluble protein mediators of the immune response.
  • Include interleukins, tumor necrosis factor (TNF), and interferon.
  • They are secreted by white blood cells and other cells and play a role in immune regulation.