Lymphatic and Immune Systems

The Lymphatic System

• Structure and Function:

  • Composed of lymphatic vessels and organs, playing a vital role in homeostasis.

  • Functions:

  • Absorbs excess interstitial fluid, returning it to the bloodstream, thereby maintaining fluid balance and preventing edema.

  • Absorbs dietary fats from the digestive system and transports them to the bloodstream through lymphatic vessels, particularly in the form of chylomicrons.

  • Helps defend the body against disease by facilitating the activity of white blood cells, which include lymphocytes that identify and eliminate pathogens.

• Lymphatic Vessels:

  • Carry lymph, which is a clear fluid containing lymphocytes, proteins, and waste products, beginning as blind-ended lymphatic capillaries in tissues.

  • Lymphatic capillaries merge into larger vessels to form two main lymphatic ducts:

  • Thoracic Duct: Collects lymph from the lower body, left arm, and left head and neck; drains into the left subclavian vein.

  • Right Lymphatic Duct: Drains lymph from the right arm and right head and neck into the right subclavian vein.

  • Similar in structure to veins, lymphatic vessels contain valves to prevent backflow; movement of lymph is predominantly reliant on the contractions of surrounding skeletal muscles and respiratory movements.

• Lymphatic Organs:

  • Primary Organs:

  • Red Bone Marrow: Primary site of blood cell production, including the maturation of B lymphocytes, directly involved in the adaptive immune response.

  • Thymus: Located in the thorax behind the sternum; it is where T lymphocytes mature and are trained to distinguish between self and non-self cells, also produces the hormone thymosin, vital for T cell development.

  • Secondary Organs:

  • Spleen: Filters blood, removes pathogens and worn-out erythrocytes; divided into white pulp (immune functionality by stimulating lymphocyte activity) and red pulp (filtering blood).

  • Lymph Nodes: Act as filtering stations where lymph is cleansed of pathogens by lymphocytes and macrophages; strategically located throughout the body to monitor and respond to infections.

  • Lymphatic Nodules: Unencapsulated clusters of lymphoid tissue found in various locations, including tonsils and Peyer patches in the small intestine.

Innate Immune Defenses

• Definition of Immunity:

  • A defense system designed to kill or remove pathogens; comprises innate (nonspecific) and adaptive (specific) immunity.

• Innate Defenses Include:

  • Physical and Chemical Barriers: Skin acts as a physical barrier, while mucous membranes trap pathogens; skin's keratin and the cilia of mucous serve as first lines of defense.

  • Inflammatory Response: Characterized by redness, heat, swelling, and pain; an immediate response dominated by neutrophils and macrophages; stimulates repair processes and local blood flow.

  • Protective Proteins: The complement system enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells; interferons are proteins that provide immunity against viral infections by signaling neighboring cells to enhance their antiviral defense mechanisms.

Adaptive Immune Defenses

• Function:

  • Activates when innate defenses are insufficient; specifically targets antigens associated with pathogens in a highly coordinated response.

• Mechanism:

  • Relies on B and T lymphocytes; these cells recognize specific foreign substances on pathogens, leading to targeted destruction.

  • B Cells (Antibody-Mediated Immunity): Responsible for producing antibodies that bind to and neutralize free antigens in body fluids, facilitating their removal by other immune cells.

  • T Cells (Cell-Mediated Immunity): Recognize and directly destroy infected host cells, contributing to the control of intracellular pathogens.

• Stages of Immune Response:

  • Involves activation of B and T cells through antigen recognition; clonal expansion results in the production of memory cells (for long-term immunity) and effector cells (for immediate response).

Acquired Immunity

• Types:

  • Active Immunity: Developed when the body produces antibodies through natural infection or vaccination, leading to long-lasting immunity.

  • Passive Immunity: Antibodies are received from external sources, such as maternal antibodies passed to infants through breast milk or placental transfer, typically short-lived.

• Role of Cytokines:

  • Signaling molecules that regulate immune responses and are crucial for communication and coordination among different immune cells, mediating inflammation and enhancing immune function.

Disorders of the Immune System

• HIV and AIDS:

  • HIV predominantly targets helper T cells (CD4+ cells), leading to a significantly weakened immune response and progressive immune deficiency known as AIDS; this condition leaves individuals vulnerable to opportunistic infections that can be life-threatening.

• Hypersensitivity Reactions:

  • These are overreactions of the immune system towards harmless antigens, causing allergic responses and other immune-mediated reactions that can impair normal physiological function.

• Autoimmune Diseases:

  • Conditions where the immune system mistakenly attacks the body’s own cells (e.g., lupus, rheumatoid arthritis). Possible triggers include genetic predisposition and environmental factors; infections are also known to sometimes trigger these diseases.

• Examples of Autoimmune Conditions:

  • Systemic Lupus Erythematosus (SLE): A multi-system disorder characterized by high levels of anti-DNA antibodies that can affect various organs, leading to systemic inflammation.

  • Myasthenia Gravis: An autoimmune disorder where antibodies interfere with communication between nerves and muscles, leading to muscle weakness; may result in life-threatening conditions such as respiratory failure.

  • Multiple Sclerosis (MS): Characterized by the immune system attacking the myelin sheath that insulates nerve fibers, leading to communication problems between the brain and the body, causing neurological dysfunctions.

Summary Tables of Antibodies.

• Classes of Antibodies:

  • Includes IgG (most abundant, crosses placenta), IgM (first response to infection), IgA (found in mucosal areas), IgD (function unclear), and IgE (involved in allergic reactions). Each class plays distinct roles in the immune response.

• Monoclonal Antibodies:

  • Identical antibodies produced by clones of a single immune cell; they are utilized for various medical testing and therapeutic applications, including targeted cancer therapies and diagnostic tools for diseases.