Lecture 2 - Immune Tissues

• Pathogens


  Microorganisms that can cause disease
• Commensals


  Organisms that benefit from a relationship with another organism without causing harm to it
• Pathogens


  • Can include viruses, bacteria, fungi and parasites
  • Have the ability to invade and multiply within the host, leading to illness
• Commensals


  • Coexist with the host organism
  • May even provide benefits, such as aiding indigestion or preventing the growth of pathogenic bacteria in the intestine
• The distinction between pathogens and commensals is important in understanding the interactions between microorganisms and the immune system, as well as in the development of strategies for preventing and treating infectious diseases
• Innate immune system


  Acts immediately, within minutes
• Adaptive immune system


  Takes days to develop
• Innate immune system


  Recognises foreign pathogens via germ-line encoded receptors
• Adaptive immune system


  Generates highly specific responses requiring sophisticated rearrangement of receptor genes
• Innate immune system


  Has no memory
• Adaptive immune system


  Generates immune memory
• Innate immune system


  Antigen non-specific
• Cells of the innate immune system


  • Often detect 'particulate' material and seek to scavenge/remove
• Cells of the adaptive immune system


  • Highly specialised and undergo expansion, maturation and differentiation to respond to specific antigens
• Macrophages


  • Induce inflammation through the release of soluble factors, efficiently phagocytose antigens, and are found in all peripheral tissues
  • Considered the first line of defense during an infection and can increase in numbers through recruitment from the periphery
• Neutrophils


  The most common white blood cells in the blood, they migrate rapidly to the site of inflammation, are phagocytic, and release granules containing toxic components to kill bacteria
• Dendritic cells


  • The interphase of innate and adaptive immunity, their main function is to activate the adaptive immune response
  • They migrate from peripheral tissues to the lymph node and present antigens to T cells
• B cells


  • Express antigen-specific receptors called antibodies, which can recognise the 3D shape of antigens, including proteins and carbohydrates
  • Involved in the production of antibodies
• T cells


  • Express T cell receptors and are highly specific, requiring sophisticated rearrangement of receptor genes
  • Involved in cell-mediated immunity and can differentiate into T helper cells and cytotoxic T cells
• Lymphocytes


  • Involved in adaptive immunity and develop in primary lymphoid organs
  • Ultimately armed in secondary lymphoid organs, where immune responses occur
• Plasma cells


  Antibody-producing cells and are found in large numbers in the mucosa-associated lymphoid tissues, with numbers exceeding those in the spleen and lymph nodes combined
• Monocytes

  Phagocytic and can differentiate into macrophages, playing a role in the clearance of pathogens and the activation of the adaptive immune response
• Embryonic haematopoiesis


  Haematopoiesis begins in the yolk sac during embryonic development
• Fetal haematopoiesis


  • Liver serves as a site for haematopoeisis shortly before birth
  • Spleen also contributes to haematopoeisis in fetal development
• Adult and infant haematopoiesis

  In adult and infant life, bone marrow in most bones is the main site for haematopoiesis
• Emergency haematopoiesis


  In adult life, emergency haematopoiesis can occur in the spleen, liver or lymph nodes in response to chronic inflammation
• Structure of the thymus


  • Consists of two lobes surrounded by a capsule
  • Trabeculae or septa are extensions of the capsule into the cortex and medulla, establishing incomplete thymic lobules where blood vessels and nerves pass
  • The cortex is the outer layer containing many immature thymocytes
  • The medulla is the inner layer containing mature T cells and Hassall's corpuscles, which are epithelial structures
• Structure of the lymphatic system


  • Lymphatic vessels, also known as lymphatics, carry lymph and cells from peripheral tissues to draining lymph nodes
  • Lymph originates as excess interstitial fluid and plasma proteins that filter out of venules and capillaries due to hydrostatic/oncotic pressure
  • Lymph capillaries originate as "closed tubes" in almost all tissues and have a capillary wall constructed of overlapping endothelial cells that respond to fluid pressure
  • Lymphatics form from joining lymph capillaries, with superficial lymphatics following superficial veins and draining into deep lymphatics
• Structure of lymph nodes

  • Pea-sized immune structures strategically placed around the body and are highly organised tissues with specific functions for each area
  • They filter lymphatics and are found at drainage points in the lymphatic vessel network
  • Lymph nodes contain follicles, which are B cell-rich areas, and paracortex, which are T cell-rich areas where T cell-dendritic cell interaction occurs
  • Cell movement to and out of lymph nodes involves naive lymphocytes entering via high endothelial venules and exiting via efferent lymphatics, while antigen-presenting cells enter via afferent lymphatics and exit via efferent lymphatics
• Overview of spleen's functions

  • The spleen is the largest lymphoid organ in the body
  • It directs immune responses to antigens in the blood
  • 95% of the blood supply filters through the spleen in 3 minutes
  • It is important for the clearance of damaged red blood cells
  • Hyposplenism or asplenia is not fatal, but predisposes to infection (especially of capsulated bacteria)
  • Splenomegaly is common in chronic inflammatory diseases such as malaria and Hodgkin's disease
• Structural components of the spleen

  • The structure of the spleen includes white and red pulp
  • The red pulp filters blood, removes old red blood cells and contains macrophages
  • The white pulp contains specific lymphoid microenvironments, including periarterioIar lymphoid sheath (PALS) where proliferating lymphocytes are found around central arterioles
• The spleen plays a crucial role in the immune system by filtering blood and facilitating immune responses
• The structure of immune organs, such as the thymus, lymph nodes, spleen and bone marrow, is directly related to their function in the immune system
• Thymus


  • A two-lobed organ in the upper mediastinum that reaches its peak size by puberty and consists of epithelial cells that house lymphocytes
  • Its function is to mature T cells, as immature T cells migrate from the bone marrow to the thymus where they multiply and begin to mature
• Lymph nodes


  • Strategically placed around the body and serve as highly organised tissues with specific functions for each area
  • They filter lymphatics, are anatomical meeting places for cells of the immune system, and are central components of host defence