Lymphoid System and Immunity Notes

The Lymphoid System

Functions of the Lymphoid System

  • Fluid Recovery

    • Reabsorbs excess water not absorbed by capillaries and returns it to bloodstream.

  • Immune Surveillance

    • Picks up foreign cells and chemicals from tissues.

    • Detects potential harmful substances and activates protective immune responses.

  • Lipid Absorption

    • Specialized lymphatic vessels called lacteals absorb dietary lipids.

Basic Constituents of the Lymphoid System

  • Lymph

    • Recovery fluid which contains lymphocytes and other substances.

  • Lymphatic Vessels

    • Transport lymph throughout the body.

  • Lymphoid Tissue

    • Composed of aggregates of lymphocytes and macrophages in various organs.

  • Lymphoid Organs

    • Structures like lymph nodes and the spleen that are encapsulated and highly structured, separated from surrounding tissues by connective tissue capsules.

Lymph Production and Composition

  • Characteristics of Lymph

    • Clear, colorless fluid, low in protein (mainly water).

    • Contains lymphocytes, macrophages, hormones, bacteria, viruses, cellular debris, and traveling cancer cells.

  • Lymph Production Process

    • Formed from capillary filtration, absorbed by lymphatic capillaries, flows through lymphatic vessels, filtered by lymph nodes, then returned to the circulatory system.

Lymphatic Capillaries

  • Characteristics

    • Composed of loose overlapping endothelial cells, no basal lamina allowing large particles to enter.

    • Tethered to surrounding tissues by anchoring filaments preventing collapse.

    • Intercellular clefts allow for unidirectional flow of lymph.

Similarities Between Lymphatic and Blood Vessels

  • Both have valves that prevent backflow.

  • Both are thin-walled and function in fluid transport within a low-pressure system.

Mechanisms Propelling Lymph Flow

  • Primary mechanisms include:

    • Rhythmic contractions of lymphatic vessels.

    • Skeletal muscle pump (muscle contraction).

    • Respiratory (thoracic) pump (pressure change during breathing).

    • One-way valves to prevent backflow.

    • Smooth muscle contractions and arterial pulsations.

Key Cell Types in Lymphatic Tissue

  • Neutrophils

    • Aggressive antibacterial leukocytes.

  • Natural Killer (NK) Cells

    • Large lymphocytes attacking infected or cancerous cells.

  • T Lymphocytes (T cells)

    • Mature in thymus, several subtypes involved in immune responses.

  • B Lymphocytes (B cells)

    • Differentiate into plasma cells to produce antibodies, mostly mature in the bone marrow.

  • Macrophages

    • Large phagocytic cells that consume cellular debris and present antigens.

  • Dendritic Cells

    • Mobile antigen-presenting cells that alert the immune system to pathogens.

MALT (Mucosa-Associated Lymphoid Tissue)

  • Found in respiratory, digestive, urinary, and reproductive tracts.

Lymphoid Organs vs. Diffuse Lymphoid Tissue and Nodules

  • Lymphoid Organs

    • Encapsulated, structured, centralized (e.g., spleen, thymus, lymph nodes).

  • Diffuse Lymphoid Tissue

    • Unencapsulated, less organized aggregations of lymphatic tissue.

Thymus Function and Structure

  • Cortex

    • Has a blood-thymus barrier protecting developing T cells from blood-borne antigens.

  • Medulla

    • Contains mature T cells, no barrier for entering blood/lymphatic vessels.

  • Function

    • Thymic epithelial cells secrete signaling molecules essential for T cell development.

Structure and Function of Lymph Nodes

  • Bean-shaped lymphatic organs with fibrous capsule, consisting of stroma of reticular connective tissues with lymphocytes.

  • Functions:

    • Cleanses lymph and acts as activation sites for T and B cells.

Tonsils

  • Types:

    • Pharyngeal (adenoids), palatine tonsils, lingual tonsils, each having lymphoid nodules and crypts to trap pathogens.

  • Function:

    • Guard against ingested/inhaled pathogens; common cause of inflammation (tonsillitis) is viral/bacterial infection.

Spleen Anatomy

  • Red Pulp:

    • Contains red blood cells, involved in blood filtration and storage.

  • White Pulp:

    • Lymphocytes and macrophages that monitor blood for foreign antigens.

Innate Immunity

Three Lines of Defense Against Pathogens

  1. First Line: Epithelial barriers (skin, mucous membranes).

  2. Second Line: Internal defenses (leukocytes, macrophages, antimicrobial proteins).

  3. Third Line: Adaptive immune response that targets specific pathogens and provides memory for future encounters.

Characteristics of Innate vs. Adaptive Immunity

  • Innate Immunity:

    • Non-specific, immediate response, lacks memory.

  • Adaptive Immunity:

    • Specific, has memory for quicker response to previously encountered pathogens.

Skin as a Barrier

  • Composed of keratin, maintains dryness, produces antibacterial secretions.

Mucous Membranes

  • Trap microbes with sticky mucous, provide defense in respiratory and digestive tracts.

Mechanisms of White Blood Cells (WBCs) Against Pathogens

  • Neutrophils: Phagocytosis, NETs formation.

  • Eosinophils: Attack parasites, degrade antigen-antibody complexes.

  • Basophils: Release chemicals aiding leukocyte action (histamines, heparins).

  • Monocytes: Differentiate into macrophages and dendritic cells post-infiltration.

Interferons

  • Produced by infected cells, signaling surrounding cells to defend against viruses, also activate NK cells.

Complement System

  • Aids pathogen destruction via inflammation, immune clearance, phagocytosis, and cytolysis.

Natural Killer (NK) Cells

  • Patrol body for tumor and infected cells, induce apoptosis through perforin and granzymes.

Fever's Benefits

  • Promotes recovery by enhancing immune response and inhibiting microbial growth.

Inflammation Signs

  • Redness, swelling, heat, pain.

Chemokines and Inflammation Initiators

  • Bradykinin, cytokines, heparin, histamines, leukotrienes, prostaglandins, and complement.

Adaptive Immunity

Defining Characteristics

  • Systematic Effect: Response affects whole body.

  • Specificity: Tailored responses to distinct pathogens.

  • Memory: Faster response to previously encountered pathogens.

Two Forms of Adaptive Immunity

  • Cellular (Cell-Mediated): Direct attack by T cells.

  • Humoral (Antibody-Mediated): Production of antibodies by B cells.

Classification of Immunity

  • Active vs. Passive: Active involves body producing antibodies; Passive involves receiving them from another source.

  • Natural vs. Artificial: Natural is through natural exposure; Artificial is through vaccination or serum.

Antigen Definition

  • Molecules that trigger an immune response.

Antibody Definitions and Classes

  • Antibodies: Proteins acting in defense.

  • Classes: IgA, IgD, IgE, IgG, IgM with varying functions.

T Cell Development and Selection

  • Positive selection ensures functional T cells; Negative selection removes self-reactive T cells.

Antigen-Presenting Cells (APCs)

  • Include dendritic cells, macrophages, and B cells, critical for T cell activation by presenting antigens.

Mechanism of APCs and MHC Proteins

  • MHC proteins display processed antigens; essential for T cell recognition.

Cellular Immunity

Classes of T Lymphocytes

  • Cytotoxic T Cells: Attack foreign and infected cells.

  • Helper T Cells: Stimulate other immune responses.

  • Regulatory T Cells: Limit immune responses to prevent excess inflammation.

  • Memory T Cells: Recall previous infections for faster responses.

Stages of Cellular Immunity

  1. Recognition: T cells recognize antigens on MHC proteins.

  2. Activation: T cells undergo activation via antigen presentation.

  3. Attack: T cells destroy infected cells, assisted by helper T cells.

Humoral Immunity

Comparison with Cellular Immunity

  • Indirect defense tagged by antibodies in response to antigens.

B Cell Activation Process

  • Antigen binds to B cell receptors resulting in B cell activation and clonal selection.

Role of TH Cells and MHC-II Proteins

  • TH cells bind to antigen-MHC-II on B cells aiding activation.

Mechanism of Antibody Action

  • Antibodies combat antigens through neutralization, complement fixation, agglutination, and precipitation.

Immune Disorders

Types of Hypersensitivity

  • Type I: Immediate; allergic reactions mediated by IgE.

  • Type II: Antibody-mediated; involves IgG or IgM.

  • Type III: Immune complex-mediated; leads to tissue damage.

  • Type