Lymphatic System and Immunity: Structure, Function, and Clinical Relevance

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Lymphatic System: Structure and Function

Overview of the Lymphatic System

The lymphatic system is a network of vessels, nodes, and organs that helps maintain fluid balance, absorbs fats, and provides immune defense. It returns excess tissue fluid to the bloodstream and filters out harmful substances.

Diagram of the lymphatic system in the torso

Lymph: Fluid in tissue spaces carrying proteins and other substances back to the blood.
Lymphatic vessels: Permit one-way movement of lymph, beginning with microscopic, blind-ended lymphatic capillaries.
Lacteals: Specialized lymphatic capillaries in the intestinal wall that transport fats into the bloodstream.

Full-body diagram of the lymphatic system and major ducts

Lymphatic Vessels and Ducts

Right lymphatic duct: Drains lymph from the right upper extremity and right side of head, neck, and upper torso.
Thoracic duct: Largest lymphatic vessel; drains lymph from about three-fourths of the body. The cisterna chyli is an enlarged pouch along its course.

Lymphatic System in Fluid Homeostasis

The lymphatic system collects excess interstitial fluid and returns it to the bloodstream, preventing tissue swelling (edema) and maintaining blood volume.

Diagram showing lymphatic and blood capillary networks and lymph flow

Lymphatic Disorders

Lymphedema and Related Conditions

Lymphedema is swelling caused by blockage of lymphatic vessels. It can result from infection, surgery, or parasitic infestation.

Lymphangitis: Inflammation of lymphatic vessels, which may progress to septicemia.
Elephantiasis: Severe lymphedema of limbs due to parasite infestation.

Photograph of lymphedema in the lower limb Photograph of lymphangitis in the leg Photograph of elephantiasis in the leg

Lymphoid Organs

Lymph Nodes

Lymph nodes are small, bean-shaped structures that filter lymph and are located in clusters along lymphatic vessels. They contain lymphoid tissue, which is rich in lymphocytes and other immune cells.

Functions: Defense (filtering pathogens) and white blood cell (WBC) formation.
Lymph flows into nodes via afferent vessels and exits via a single efferent vessel.
Lymphadenitis: Swelling and tenderness of lymph nodes, often due to infection.
Metastasis: Cancer cells can spread through lymphatic vessels to other body parts.

Diagram of lymph node function in infection Lymphangiogram showing lymph nodes and vessels in the pelvis

Lymphatic Drainage of the Breast

The breast contains an extensive network of lymphatic vessels and nodes, which is clinically significant in the spread of breast cancer and in post-surgical complications such as lymphedema.

Diagram of lymphatic drainage of the breast Clinical application: Lymphedema after breast surgery

Thymus

The thymus is a lymphoid organ located in the mediastinum. It is essential for the development of T lymphocytes (T cells) and the secretion of thymosins, hormones that regulate T-cell maturation. The thymus undergoes involution (replacement by fat) after childhood.

Tonsils

Tonsils are masses of lymphoid tissue located around the openings of the mouth and throat. They include the palatine, pharyngeal (adenoids), and lingual tonsils. Tonsils help protect against pathogens entering through the oral and nasal cavities but are prone to chronic infection and enlargement.

Diagram showing the location of tonsils in the mouth and throat

Spleen

The spleen is the largest lymphoid organ, located in the upper left abdomen. It filters blood, removes old red blood cells, and acts as a blood reservoir. Splenomegaly is enlargement of the spleen, often due to infection or disease.

Lymphoma

Lymphoma is a malignant tumor of lymph nodes. The two main types are Hodgkin disease and non-Hodgkin lymphoma, both of which cause painless lymph node enlargement and systemic symptoms.

Immunity: Innate and Adaptive Defenses

Function of the Immune System

The immune system protects the body from pathogens, foreign cells, and cancerous cells using a combination of nonspecific (innate) and specific (adaptive) mechanisms.

Innate (Nonspecific) Immunity

Innate immunity provides immediate, general defense against a wide range of pathogens. It includes physical barriers, phagocytic cells, and chemical mediators.

Innate Immunity	Adaptive Immunity
Synonyms: Nonspecific, native, genetic immunity	Synonyms: Specific, acquired immunity
Not specific—recognizes variety of nonself or abnormal cells and particles	Specific—recognizes only specific antigens on certain cells or particles
Rapid—immediate up to several hours	Slower—several hours to several days
No memory—same response to repeated exposures	Yes—enhanced response to repeated exposures
Chemicals: Complement proteins, interferons, others	Chemicals: Antibodies, various signaling chemicals
Cells: Phagocytes (neutrophils, macrophages, dendritic cells)	Cells: Lymphocytes (B cells and T cells)

Table comparing innate and adaptive immunity

Mechanisms of Innate Defense

Mechanism	Description
Mechanical and chemical barriers	Physical impediments to entry of foreign cells or substances
Skin and mucous membranes	Continuous wall separating internal and external environments
Secretions	Substances like sebum, mucus, acids, and enzymes inhibit pathogens
Inflammation	Attracts immune cells, increases blood flow and vascular permeability
Fever	Enhances immune reactions
Phagocytosis	Ingestion and destruction of pathogens by cells
Neutrophils	Short-lived phagocytes
Macrophages	Long-lived phagocytes, often antigen-presenting cells
Natural killer (NK) cells	Destroy any cell lacking a normal self-antigen
Complement	Protein cascade that produces cell lysis
Interferon (IF)	Proteins released by virus-infected cells to inhibit viral replication

Table of mechanisms of innate defense

Inflammatory Response

Inflammation is a key innate defense mechanism, characterized by heat, redness, pain, and swelling. It attracts immune cells, increases blood flow, and promotes healing.

Flowchart of the inflammatory response

Adaptive (Specific) Immunity

Adaptive immunity is the body's ability to recognize, respond to, and remember specific harmful substances. It involves B and T lymphocytes and provides long-lasting protection.

Inherited immunity: Present from birth.
Acquired immunity: Developed after exposure to antigens.

Types of Acquired Immunity

Type	How Acquired	Example
Natural Active	Exposure to disease	Recovery from infection
Natural Passive	Mother to child (placenta, milk)	Maternal antibodies
Artificial Active	Vaccination	Immunization shots
Artificial Passive	Injection of antibodies	Antiserum administration

Immune System Molecules

Cytokines: Signaling proteins that regulate immune responses.
Antibodies: Proteins that bind specific antigens, forming antigen-antibody complexes. Functions include neutralizing toxins, agglutinating cells, and promoting phagocytosis.

Diagram of antibody functions

Complement Proteins and Cascade

Complement proteins are a group of plasma proteins that, when activated, enhance immune responses by lysing foreign cells and promoting inflammation.

Activation occurs when antibodies bind antigens, exposing complement-binding sites.
The complement cascade results in the formation of protein rings that create holes in foreign cell membranes, causing cell lysis.

Diagram of the complement cascade and cell lysis

Immune System Cells

Phagocytes

Phagocytes ingest and destroy foreign substances. Major types include neutrophils, monocytes (which become macrophages), and dendritic cells (DCs). Macrophages and DCs also act as antigen-presenting cells (APCs).

Diagram of the phagocytosis process Cycle of phagocytosis in a macrophage Electron micrograph of a dendritic cell Description of dendritic cell as an APC

Lymphocytes

Lymphocytes are the most numerous immune cells and include natural killer (NK) cells, B cells, and T cells. NK cells are part of innate immunity, while B and T cells are central to adaptive immunity.

Natural Killer (NK) Cells: Destroy virus-infected and tumor cells without prior exposure.
B Cells: Develop in bone marrow, produce antibodies (humoral immunity), and form plasma and memory cells.
T Cells: Develop in the thymus, mediate cell-mediated immunity, and differentiate into cytotoxic, helper, and regulatory T cells.

Immune System Disorders

Hypersensitivity

Hypersensitivity is an excessive or inappropriate immune response, such as allergies (immediate or delayed) and contact dermatitis.

Autoimmunity

Autoimmunity is an immune response against self-antigens, leading to diseases like systemic lupus erythematosus (SLE).

Alloimmunity

Alloimmunity (isoimmunity) is an excessive reaction to antigens from another individual, important in pregnancy and tissue transplants.

Immune Deficiency

Congenital immune deficiency: Present at birth, e.g., severe combined immune deficiency (SCID).
Acquired immune deficiency: Develops after birth, e.g., AIDS caused by HIV infection of T cells.

Review Questions

What is the role of lymph nodes in the body?
Describe the difference between afferent and efferent lymphatic vessels.
Why is the thymus important for immunity?
What are the two principal categories of lymphoma?
What is the difference between adaptive and innate immunity?
Describe the difference between active and passive immunity.