Lymphatic System & Immunity: Functions, Pathways, and Defense Mechanisms

Lymphatic Pathways

Lymphatic pathways begin as lymphatic capillaries, which come together to form afferent lymphatic vessels, which lead to lymph nodes.

Efferent Lymphatic Vessels

The vessels that leave the lymph nodes are called efferent lymphatic vessels, which come together to form lymphatic trunks.

Lymphatic Capillaries

Microscopic closed-ended tubes that extend into interstitial spaces and receive lymph through their thin walls.

Anchoring Filaments

Structures associated with lymphatic capillaries that serve an important function during edema.

Locations of Lymphatic Capillaries

Lymphatic capillaries are located throughout the body, except in avascular tissues, central nervous system, splenic pulp, and bone marrow.

Lacteals

Lymphatic capillaries within villi of the small intestine.

Lymphatic Vessels (LV)

Formed by the merging of lymphatic capillaries and have walls similar to veins, possessing valves that prevent backflow of lymph.

Afferent Lymphatic Vessels

Lymphatic vessels that lead to lymph nodes.

Lymphatic Trunks

Structures that are formed from the merging of efferent lymphatic vessels.

Collecting Ducts

Ducts that lymphatic trunks lead to before joining the subclavian veins.

Subclavian Veins

Veins where lymph enters the cardiovascular system.

Tissue Fluid

Fluid that forms from the interstitial spaces and is collected by lymphatic capillaries.

Lymph

Fluid that is collected by lymphatic capillaries and transported through lymphatic vessels.

Innate Defenses

Nonspecific body defense mechanisms that provide immediate protection against pathogens.

Adaptive Defenses

Specific body defense mechanisms that develop over time and provide targeted responses to pathogens.

Types of Lymphocytes

Two major types of lymphocytes are formed and activated to function in immune mechanisms.

Antibody Molecule

Structure that consists of parts which are identified for their role in immune response.

Types of Antibodies

Five types of antibodies that have distinct actions in the immune response.

Primary Immune Response

The initial response of the immune system to a specific antigen.

Secondary Immune Response

The more rapid and effective response of the immune system upon subsequent exposure to the same antigen.

Active Immunity

Immunity that develops as a result of exposure to an antigen and the subsequent immune response.

Passive Immunity

Immunity that is acquired through the transfer of antibodies from another source.

Principal lymphatic trunks

Include lumbar trunk, intestinal trunk, bronchomediastinal trunk, subclavian trunk, jugular trunk, and intercostal trunk.

Right lymphatic duct

Drains the right upper body (25% of total body).

Thoracic (left lymphatic) duct

Drains the remaining 75% of the body's lymph.

Constituents of plasma

Primarily water, dissolved substances including small plasma proteins, nutrients, wastes, gases, electrolytes, enzymes, and hormones.

Lymph Formation

As protein concentration in interstitial spaces increases, its pressure increases, forcing tissue fluid into lymphatic capillaries.

Lymph flow

Under low pressure and may not flow readily without aid from external forces.

Skeletal muscles

Aid movement of lymph through their squeezing action.

Low pressure in the thoracic cavity

Created by breathing movements, moves lymph up from abdominal to thoracic region.

Obstruction of lymph movement

Any condition that interferes with the flow of lymph results in edema.

Edema

Accumulation of excess interstitial fluid leading to swelling of tissues.

Surgical removal of lymph nodes

Causes obstruction and results in edema.

Lymph Function

Returns small leaked plasma proteins back to the bloodstream, transports foreign particles to the lymph nodes, and transports lipids and lipid-soluble vitamins absorbed in GI tract to bloodstream.

Lymph Nodes

Located along lymphatic pathways; contain lymphocytes and macrophages, which destroy invading microorganisms.

Size of lymph nodes

Usually less than 2.5 cm, with a bean-like shape.

Hilum

Indented region of the lymph node where blood vessels, nerves, and efferent lymphatic vessels are attached.

Dense CT capsule

Encloses the lymph node and extends into the node, subdividing it into nodules.

Cortex of lymph nodes

Outer region containing germinal centers of densely packed B cells and macrophages in lymphatic nodules.

Inner region

Medulla contains T cells (+ macrophages and plasma cells) arranged as medullary cords (spaces through which lymph flows).

Flow of Lymph through Lymph Node

Lymph enters the node through one of several afferent lymphatic vessels on convex surface.

Locations of Lymph Nodes

Lymph nodes generally occur in groups or chains along the paths of larger lymphatic vessels.

Functions of Lymph Nodes

Removal and destruction of potentially harmful foreign particles from lymph.

Thymus

Soft, bilobed organ located within the mediastinum.

Thymus Lobule Structure

Each lobule contains an outer (dark-staining) cortex filled with densely packed lymphocytes around a central medulla (light staining) filled with swirled epithelial cells.

Thymus Function

Immature (undifferentiated) T cells migrate from the bone marrow to the thymus via the blood.

Spleen

Located in the upper left portion of the abdominal cavity (behind stomach).

Spleen Structure

Resembles a large lymph node that is encapsulated and subdivided into lobules by connective tissue.

Spleen Tissue Types

Contains white pulp = lymphocytes arranged around central arteries and red pulp = blood filled sinuses (venous blood that also serves as blood reservoir).

Spleen Functions

Removal and destruction of foreign particles and worn blood cells from blood.

Spleen Function

Macrophages remove and destroy bacteria and damaged or worn red blood cells and platelets through phagocytosis.

Infection

Caused by the presence and multiplication of pathogens.

Pathogens

Viruses and microorganisms (bacteria, fungi, protozoans, parasites) that cause disease.

Innate (nonspecific) resistance

1st and 2nd lines of defense.

Adaptive (specific) resistance

3rd line of defense.

Innate (nonspecific) defenses

Protection against a wide range of pathogens.

Species Resistance

Each species is resistant to certain diseases that may affect other species due to lack of receptors for the pathogen or unsuitable environmental conditions.

Mechanical barriers

First line of defense that includes the skin and mucous membranes.

Inflammation

A tissue response to damage, injury, or infection.

Localized tissue redness

Also known as rubor, a response in inflammation.

Swelling

Also known as tumor, a response in inflammation.

Heat

Also known as calor, a response in inflammation.

Pain

Also known as dolor, a response in inflammation.

Pus

May form as white blood cells, bacterial cells, and debris accumulate.

Clot (fibrin)

May form in affected tissues during inflammation.

Fibroblasts

Cells that arrive at the site of injury during inflammation.

Chemical Barriers

First and second lines of defense that include various substances.

Enzymes in gastric juice

Pepsin is lethal to many pathogens.

Enzyme in tears

Lysozyme has antibacterial action.

Acid

Low pH in stomach (hydrochloric acid) prevents growth of some bacteria.

Salt

High salt concentration in perspiration kills some bacteria.

Defensins

Destroy bacteria by making holes in their cell walls and/or membranes.

Collectins

Protect by attaching themselves to a variety of microbes and provide broad protection against them.

Complement

A system of 11 proteins that work to lyse infected cells and attract phagocytes to the area.

Interferons

Hormone-like peptides produced by certain uninfected cells in response to viruses that interfere with virus proliferation and stimulate phagocytosis.

Phagocytosis

The process by which specialized cells engulf and ingest foreign particles to destroy them.

Neutrophils

The most active phagocytes in the blood.

Monocytes

Another type of active phagocyte in the blood.

Reticuloendothelial tissue

Phagocytic cells associated with the linings of blood vessels in the bone marrow, liver, spleen, and lymph nodes.

Phagocytes

Remove and destroy foreign particles from tissues and body fluids.

Fever

Infection by bacteria and viruses causes some lymphocytes to produce Interleukin I, which increases body temperature.

Body temperature increase factors

Other factors that can also increase body temperature include exposure to heat, UV light, acids, and bases.

Increased body temperature effect

Decreases blood iron levels, which increases phagocytic activity.

Adaptive (Specific) Defenses

Protection against particular disease-causing agents; it is our third line of defense against infection.

Antigens (Ag's)

A substance (usually a protein) that causes the formation of an antibody and reacts specifically with that antibody.

Self proteins

Body cells inventory proteins and other large molecules present in the body before birth.

Lymphocyte receptors

Develop after the inventory to differentiate between foreign (non-self) antigens and self-antigens.

Immune response (IR)

Stimulated when non-self or foreign antigens enter human tissues and combine with T & B cell surface receptors.

Lymphocyte origins

Lymphocytes originate in red bone marrow and are released into the blood before they become differentiated.

T cells

Processed from undifferentiated lymphocytes in the thymus; constitute 70%-80% of circulating lymphocytes.

B cells

Processed from undifferentiated lymphocytes in the bone marrow; constitute 20%-30% of circulating lymphocytes.

Antigen-Presenting Cells

Begin the immune response; a macrophage is typically the first cell to respond to an antigen.

Macrophage function

Alerts lymphocytes to the invader after digesting the antigen.

Major histocompatibility complex (MHC)

Codes for the self-protein that attaches a copy of the foreign antigen to the cell membrane of the macrophage.

Lymphocyte activation

Occurs when a lymphocyte recognizes and binds to the antigen-presenting cell.

Cell-mediated immunity

T cells respond to antigens directly by cell-to-cell contact.

Cytokines

Substances secreted by T cells to enhance other immune responses to antigens.

Colony stimulating factors

Stimulate bone marrow to produce lymphocytes.

Interleukins

Control lymphocyte differentiation.

Tumor necrosis factor

Stops tumor growth.