Lymphatic System and Immunity: Structured Study Notes

Lymphatic System

Structures and Functions 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 consists of lymphatic vessels, lymph nodes, and specialized organs such as the thymus, tonsils, and spleen.

• Lymphatic vessels: Transport lymph, a fluid containing proteins and other substances, back to the bloodstream.

• Lymph nodes: Filter lymph and provide sites for immune cell development.

• Lymph: Formed from interstitial fluid that enters lymphatic capillaries; returns proteins and excess fluid to the blood.

• Main ducts: The right lymphatic duct drains the right upper body; the thoracic duct drains most of the body and includes the cisterna chyli for lymph storage.

• Lacteals: Specialized lymphatic capillaries in the intestinal wall that absorb fats.

Lymphatic System in Fluid Homeostasis

The lymphatic system prevents the accumulation of excess tissue fluid by draining fluid that is not reabsorbed by blood vessels. Lymph nodes filter this fluid before it returns to the bloodstream.

• Interstitial fluid: Fluid between tissue cells; becomes lymph when it enters lymphatic vessels.

• One-way flow: Valves in lymphatic vessels ensure lymph moves only toward the heart.

Lymphedema and Related Disorders

Lymphedema is swelling caused by blockage of lymphatic vessels. It can result from infection, inflammation, or parasitic infestation (elephantiasis).

• Lymphangitis: Inflammation of lymphatic vessels, often visible as red streaks.

• Elephantiasis: Severe lymphedema due to parasitic worms, causing massive swelling.

Lymphoid Organs

Lymph Nodes

Lymph nodes are clusters of lymphoid tissue that filter lymph and provide immune defense. They are located along lymphatic vessels and are essential for white blood cell formation.

• Flow: Lymph enters 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.

Lymphangiogram

A lymphangiogram is an imaging technique using dye to visualize lymphatic vessels and nodes, often used to assess lymphatic drainage and detect blockages or tumors.

Lymphatic Drainage of the Breast

The breast is drained by a network of lymphatic vessels and nodes, which is clinically important in breast cancer therapy and risk of lymphedema after node removal.

Tonsils

Tonsils are masses of lymphoid tissue located around the mouth and throat, serving as the first line of defense against pathogens entering through the oral and nasal cavities.

• Palatine tonsils: Located on each side of the throat.

• Pharyngeal tonsils (adenoids): Near the posterior opening of the nasal cavity.

• Lingual tonsils: Near the base of the tongue.

Immune System

Function of the Immune System

The immune system protects the body from pathogens, foreign tissue cells, and cancerous cells. It consists of defensive cells and molecules rather than organs.

• Nonspecific immunity: Includes barriers like skin and mucous membranes.

• Specific immunity: Involves recognition and memory of specific pathogens.

Innate Immunity

Innate (nonspecific) immunity provides rapid defense against a wide range of pathogens. It includes physical barriers, phagocytic cells, and the inflammatory response.

• Inflammation: Attracts immune cells, increases blood flow and permeability, and promotes movement of white blood cells to sites of injury.

• Signs: Heat, redness, pain, and swelling.

Adaptive Immunity

Adaptive (specific) immunity allows the body to recognize, respond to, and remember specific harmful substances. It can be inherited or acquired, and involves memory cells for rapid response upon re-exposure.

• Natural immunity: Acquired through natural exposure.

• Artificial immunity: Acquired through deliberate exposure (e.g., vaccination).

• Active immunity: Results from direct exposure to antigens.

• Passive immunity: Results from transfer of antibodies (e.g., mother to child).

Immune System Molecules

Key molecules include cytokines and antibodies. Antibodies bind to antigens, forming antigen-antibody complexes that neutralize toxins, agglutinate cells, and promote phagocytosis.

• Cytokines: Signaling proteins used by immune cells for communication (e.g., interleukins).

• Antibodies: Protein compounds with specific combining sites for antigens.

Complement Proteins

Complement proteins are present in blood in an inactive state. When activated, they trigger a cascade of reactions that create holes in the plasma membrane of foreign cells, leading to cell lysis.

• Complement cascade: Series of reactions resulting in destruction of pathogens.

• Other roles: Attract immune cells, activate cells, mark cells for destruction, increase blood vessel permeability.

Immune System Cells

The immune system includes phagocytes and lymphocytes, each with specialized functions.

• Phagocytes: Ingest and destroy foreign cells (neutrophils, monocytes/macrophages, dendritic cells).

• Lymphocytes: Most numerous immune cells; include B cells and T cells.

• Natural Killer Cells: Destroy infected or abnormal cells.

B-Cell Development and Function

B cells develop in the bone marrow and are responsible for humoral immunity. Upon activation by antigens, B cells differentiate into plasma cells (which secrete antibodies) and memory cells (which provide long-term immunity).

• Plasma cells: Secrete antibodies into the blood.

• Memory cells: Enable rapid response upon re-exposure to the same antigen.

T-Cell Development and Function

T cells develop in the thymus and are responsible for cell-mediated immunity. They include cytotoxic T cells (kill infected/tumor cells), helper T cells (activate other immune cells), and regulatory T cells (suppress immune responses).

• Cytotoxic T cells: Release substances that poison infected or tumor cells.

• Helper T cells: Activate macrophages and B cells.

• Regulatory T cells: Suppress immune responses.

Hypersensitivity, Autoimmunity, and Alloimmunity

Hypersensitivity is an excessive immune response, such as allergies. Autoimmunity is an inappropriate response to self-antigens, causing diseases like systemic lupus erythematosus (SLE). Alloimmunity is a reaction to antigens from another human, such as in tissue transplants.

• Allergy: Immediate or delayed responses to harmless environmental antigens.

• Autoimmunity: Immune response against self-antigens.

• Alloimmunity: Immune response to antigens from another person (e.g., transplant rejection).

Immune System Deficiency

Immune deficiencies can be congenital (e.g., SCID) or acquired (e.g., AIDS). They result from improper development or destruction of immune cells, leading to increased susceptibility to infections.

• SCID: Severe combined immune deficiency due to stem cell disruption.

• AIDS: Acquired immunodeficiency syndrome caused by HIV infection of T cells.

Summary Table: Types of Immunity

Type	Source	Duration	Example
Natural Active	Exposure to pathogen	Long-term	Measles infection
Natural Passive	Maternal antibodies	Short-term	Placental transfer
Artificial Active	Vaccination	Long-term	Flu vaccine
Artificial Passive	Injection of antibodies	Short-term	Antivenom