Chapter 14: The Lymphatic System and Immunity

Overview

This chapter explores the structure and function of the lymphatic system, the production and flow of lymph, the roles of lymphoid tissues and organs, and the mechanisms of innate and adaptive immunity. Understanding these concepts is essential for comprehending how the body defends itself against disease and maintains homeostasis.

General Functions of the Lymphatic System

• Fluid Balance: Returns excess interstitial fluid to the bloodstream, preventing tissue swelling (edema).

• Fat Absorption: Absorbs dietary fats and fat-soluble vitamins from the digestive tract via specialized lymphatic vessels called lacteals.

• Immune Defense: Provides sites for immune surveillance and response, filtering pathogens and foreign particles from lymph and blood.

Production, Function, and Transport of Lymph

• Production: Lymph forms when interstitial fluid enters lymphatic capillaries.

• Function: Transports immune cells, removes waste, and returns proteins and excess fluid to the blood.

• Transport (Flow): Lymph flows through lymphatic vessels, passing through lymph nodes, and eventually drains into the venous system via the thoracic duct and right lymphatic duct.

Comparison of Lymph, Plasma, and Interstitial Fluid

Components and Structures of the Lymphatic System

• Lymphoid tissues and organs. Lymphoid tissues are collections of loose connective tissue and lymphocytes in structures called lymphoid nodules.

• Fluid. A fluid called lymph flows through the lymphatic vessels. Lymph resembles plasma but contains a much lower concentration of proteins.

• Lymphocytes. Lymphocytes are specialized white blood cells with an array of specific functions in defending the body.

• Lymphoid tissues and organs. Lymphoid tissues are collections of loose connective tissue and lymphocytes in structures called lymphoid nodules.

Functions of the lymphatic system

• Production, maintenance, and distribution of lymphocytes.

• Return of fluid and solutes from peripheral tissues to the blood.

• Distribution of hormones, nutrients, and waste products from their tissues of origin to the general circulation.

Describe how the structure of lymphatic capillary system.

• the smallest lymphatic vessels.

• They begin as blind pockets in peripheral tissues.

• The region of overlap acts as a one-way valve. It permits fluids and solutes (including those as large as proteins), along with viruses, bacteria, and cell debris, to enter, but it prevents them from returning to the intercellular spaces.

How is the lymph returned to the lymphatic system?

• The thoracic duct collects lymph from the lower abdomen, pelvis, and lower limbs, and from the left half of the head, neck, and chest. It empties its collected lymph into the venous system near the junction of the left internal jugular vein and the left subclavian vein.

• The smaller right lymphatic duct ends at a comparable location on the right side. It delivers lymph from the right side of the body above the diaphragm. It empties into the right subclavian vein.

Lymphocytes: T Cells, B Cells, and NK Cells

• T Cells: (thymus-dependent) cells. Mediate cellular immunity; attack infected or abnormal cells directly.

• B Cells: (bone marrow–derived) cells make up 10–15 percent of circulating lymphocytes. Produce antibodies for humoral immunity; differentiate into plasma cells.

• Natural Killer (NK) Cells: Provide rapid, non-specific defense against infected or cancerous cells.

General Structure of Lymphoid Tissue

• Densely packed lymphocytes in loose connective tissues

• Protect epithelia in body systems open to the external environment

MALT (Mucosa-Associated Lymphoid Tissue)

• Definition: Aggregates of lymphoid tissue found in mucous membranes throughout the body.

• Function: Protects mucosal surfaces (e.g., digestive, respiratory, urinary tracts) from pathogen entry.

Lymph Node Concentration and Structure

• Concentrated in cervical (neck), axillary (armpit), and inguinal (groin) regions.

• Many afferent vessels allow lymph to enter from multiple directions, while a single efferent vessel ensures slow filtration and immune cell exposure.

Thymus

• Location: In the superior mediastinum, posterior to the sternum, anterior to the heart.

• Function: Site of T cell maturation and selection; most active during childhood.

Spleen

• Location: In the left upper quadrant of the abdomen, lateral to the stomach. Between stomach and left kidney.

• Function: Filters blood, removes old erythrocytes, stores platelets, and initiates immune responses to blood-borne antigens.

Innate vs. Adaptive Immunity

• Innate Immunity: Non-specific, immediate defense mechanisms present at birth (e.g., barriers, phagocytes).

• Adaptive Immunity: Specific, acquired defense involving lymphocytes (T and B cells) and memory formation.

Innate Defenses

Adaptive Defenses

Branches of Adaptive Immunity

• Cell-Mediated Immunity: Involves T cells that directly attack infected or abnormal cells.

• Humoral (Antibody-Mediated) Immunity: Involves B cells that produce antibodies targeting specific antigens.

Forms of Immunity

• Active Immunity: Develops after exposure to antigen; can be natural (infection) or artificial (vaccination).

• Passive Immunity: Transfer of antibodies from another source; can be natural (maternal antibodies) or artificial (injection of antibodies).

T Cells

• Activation: T cells are activated when their receptors bind to antigen fragments presented by antigen-presenting cells (APCs) with major histocompatibility complex (MHC) molecules.

• Function after Activation: Differentiate into cytotoxic T cells (destroy infected cells), helper T cells (stimulate other immune cells), or memory T cells (provide long-term immunity).

B Cells

• Activation: B cells are activated when they encounter their specific antigen and receive help from helper T cells.

• Function after Activation: Differentiate into plasma cells (produce antibodies) and memory B cells (provide long-term immunity).

Abnormal Immune Responses (Homeostatic Imbalances)

• Autoimmune Disorders: The immune system attacks the body’s own tissues (e.g., rheumatoid arthritis, lupus).

• Immunodeficiency: Inadequate immune response, increasing susceptibility to infections (e.g., HIV/AIDS, congenital immunodeficiencies).

• Hypersensitivity Reactions: Exaggerated immune responses to harmless antigens (e.g., allergies, asthma).