Lymphatic and Immune System Overview

Lymphatic System and Immune System

• The lymphatic system is a network of vessels, cells, and organs that transports excess fluids back to the bloodstream and filters pathogens. It works closely with the immune system.
• Lymph carrying antibodies and immune cells collects in the thoracic duct before returning to the blood circulatory system.
• The immune system cells circulate in blood, reside in tissues, and are found in both blood and tissues.

Immune System Cells Circulating in Blood

• Neutrophils: The most abundant circulating white blood cells; contain granules with enzymes (lysozymes, collagenase, elastase, defensins) that digest phagocytosed material; first responders to bacterial infections.
• Eosinophils: Found in blood and tissues; have a two-lobed nucleus and granular cytoplasm; granules release contents damaging parasites and contributing to allergic reactions.
• Basophils: Have a bilobed nucleus and basophilic granules containing histamine and enzymes (peroxidases, hydrolases); involved in allergic reactions and parasitic infections.

Organs of the Immune System

• Primary (Central) Lymphoid Organs: Bone marrow and thymus; sites of lymphocyte development and maturation.
• Bone Marrow: Contains pluripotent stem cells that differentiate into blood cells, including immune cells; B lymphocytes mature here.
• Thymus: Site of T lymphocyte maturation; T cells learn to distinguish self from non-self.
• Secondary (Peripheral) Lymphoid Organs: Spleen, lymph nodes, and mucosa-associated lymphoid tissue (MALT); sites of lymphocyte activation and initiation of adaptive immune responses.
• Spleen: Largest secondary lymphoid organ; filters blood, removing foreign material and damaged cells; contains lymphocytes and phagocytes.
• Lymph Nodes: Small, bean-shaped organs throughout the body; filter lymph, removing foreign material; contain lymphocytes and phagocytes.
• MALT: Diffuse lymphoid tissue in mucosal linings (intestinal, respiratory, genitourinary tracts); contains lymphocytes and phagocytes. It represents over 50% of the body's lymphoid tissue.

Immune System Cells Residing in Tissues

• Macrophages: Phagocytic cells in various tissues; ingest and kill pathogens; produce inflammatory cytokines; clear dead tissue and initiate repair.
• Dendritic Cells: Found in skin and mucous membranes; phagocytic; migrate to lymph nodes or spleen to activate T lymphocytes.
• Mast Cells: Located in tissues (skin, mucosa); similar to basophils; granules contain mediators involved in inflammation and allergic reactions.

Lymphocytes

• B Lymphocytes: Produce antibodies; involved in toxin and virus neutralization and stimulating phagocytosis. Mature into plasma cells.
• T Lymphocytes: Two main types:
  • T Helper Cells: Secrete cytokines; help other immune cells.
  • T Cytotoxic Cells: Kill malignant, virally infected, or intracellular bacteria-infected cells.
• Natural Killer (NK) Cells: Large, granular lymphocytes; destroy malignant and virally infected cells. Increased counts are seen in viral infections.
• Monocytes: Circulate in blood; phagocytic; differentiate into macrophages in tissues.

Branches of the Immune System

• Innate Immunity: First line of defense; nonspecific; no memory; rapid response.
• Adaptive Immunity: Specific; has memory; slower response; activated when innate immunity fails. It distinguishes between self and non-self.

Innate vs. Adaptive Immunity

• Specificity: Innate immunity is non-specific, reacting to common microbial structures. Adaptive immunity is highly specific, targeting the specific microbe.
• Memory: Innate immunity lacks memory; adaptive immunity has immunological memory, resulting in a faster and stronger response upon re-exposure to the same pathogen.