Lymphatic/ Immune System

• The lymphatic system has three main functions:

  • Return interstitial fluids and proteins back into circulation (leaked out of capillaries)
  • Provide the structural basis for the immune system
  • Transport dietary lipids
• The lymphatic system consists of three parts:

  • Lymphatic fluid
  • Lymph vessels
  • Lymph organs and tissues
• Lymphatic flow involves lymph capillaries that are very permeable (more than blood capillaries) and are found in all body tissues except bone, teeth, bone marrow, and the central nervous system
• Lacteals are specialized lymph capillaries that drain the small intestine of dietary lipids
• In the lymphatic flow, the collecting duct is where all the capillaries empty into, uniting to form either the thoracic duct or the right lymphatic duct
• The thoracic duct empties into the left subclavian vein and carries lymph from the lower body, left upper body, and left side of the head and neck
• The right lymphatic duct empties into the right subclavian vein and carries lymph from the right side of the head and neck and the right side of the thorax
• Lymph is transported unidirectionally from the tissues toward the heart with the help of the respiratory pump and skeletal muscle pump
• Primary lymph organs are sites where stem cells become immunocompetent and capable of creating an immune response, including the red bone marrow and thymus
• Secondary lymph organs/tissues are sites where most immune responses occur, such as lymph nodes, spleen, and tonsils
• The thymus is one of the primary lymphoid organs, where lymphocyte maturation occurs, and it grows in childhood while the immune system is maturing, but atrophies in adolescence
• The thymus cortex is composed of a large number of T cells, while the medulla is where mature lymphocytes enter the bloodstream
• Lymph nodes are the principal lymphoid organs of the body, arranged in clusters along lymphatic vessels, with two regions: cortex and medulla
• The function of lymph nodes includes filtering lymph, destroying pathogens by macrophages, and activating lymphocytes to attack antigens in the body
• Lymph flow in lymph nodes involves afferent vessels bringing fluid to the nodes, subscapular and medullary sinuses, and efferent vessels allowing lymphocytes and macrophages time to destroy pathogens
• The spleen is the largest lymphoid organ of the body, with red pulp dealing with blood cells and white pulp dealing with lymphocytes and immune function
• Tonsils are the simplest lymphoid organs, with different types like palatine tonsils, lingual tonsils, and pharyngeal tonsils (adenoids)
• Other specialized lymphoid tissues include Mucosa-Associated Lymphoid Tissue (MALT) and Peyer's Patches
• The immune system is defined as resistance to disease, with innate and adaptive branches working together
• The innate defense system includes surface barriers, phagocytes, natural killer cells, chemical mediators, inflammation, and fever
• Phagocytes, like macrophages and neutrophils, play a crucial role in the innate defense system by engulfing and destroying pathogens
• The process of phagocytosis involves chemotaxis, adherence, ingestion, digestion, and killing of pathogens
• Natural killer cells are specialized lymphocytes that target cells lacking "self" cell-surface receptors, inducing apoptosis in abnormal cells
• Chemical mediators like interferons and complement proteins play a role in the innate defense system by protecting against pathogens
• Complement proteins circulate in an inactive form and when activated, they enhance phagocytosis and create a Membrane Attack Complex (MAC) to kill bacteria and infected cells
• Inflammation is triggered by tissue injury or infection and aims to prevent the spread of pathogens, dispose of cell debris, and set the stage for repair
• Inflammation causes cardinal signs like pain, redness, swelling, and heat, with substances like histamine, kinins, and prostaglandins contributing to the process
• Fever is a systemic response to invading microorganisms, caused by pyrogens that reset the body's thermostat to increase metabolic rate and speed up repair
• The adaptive defense system uses lymphocytes and other molecules to identify and eliminate specific pathogens, working in coordination with the innate defense system
• Adaptive Defense in multicellular organisms:

  • Requires specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen due to a higher surface area to volume ratio
• Adaptive Defense uses lymphocytes and other molecules to identify and destroy non-self substances
• Adaptive Defense involves two branches:

  • Humoral (Antibodies)
  • Cellular (Cell Mediated)
  • Both systems recognize antigens and provoke an immune response
• Types of antigens in Adaptive Defense:

  • Complete Antigens: Foreign proteins with immunogenicity and reactivity
  • Incomplete Antigens (HAPTENS): Small molecules that become immunogenic only when attached to body proteins, leading to allergies
• Adaptive Defense uses two types of lymphocytes:

  • B lymphocytes (mature in red bone marrow for humoral immunity)
  • T lymphocytes (mature in thymus for cell-mediated immunity)
  • Mature lymphocytes have immunocompetence and are self-tolerant
• Humoral Immunity Response:

  • Uses B cells for antigen challenge and antibody production
  • Leads to the formation of memory cells for immunological memory
• Humoral Immunity Response:

  • Primary immune response: 3-6 days for antibodies to form, peak in 10 days
  • Secondary immune response: Memory cells respond within hours, peak antibodies within 2-3 days
• Humoral Immunity:

  • Active humoral immunity involves B cells encountering antigens and producing antibodies
  • Passive humoral immunity involves receiving antibodies produced outside the body
• Humoral Immunity:

  • Antibodies/Immunoglobulins are proteins secreted by plasma cells
  • Consist of 4 protein chains (2 heavy, 2 light) with variable and constant regions
• Humoral Immunity:

  • 5 classes of Antibodies: IgM, IgA, IgD, IgG, IgE
  • Monoclonal antibodies (MABs) are used in research, clinical testing, and cancer treatment
• Cell-Mediated Immunity:

  • T cells defend against intracellular antigens and abnormal cells
  • Activated T cells differentiate self vs. non-self antigens