Organs Tissues and Cell of the Immune System

• Lymphatic system


  System of lymphatic vessels and organs that drain fluid from the tissues, transport antigens and lymphocytes from the tissues to organised lymphoid tissues where the antigens are trapped and the lymphocytes interact with the trapped antigens and undergo activation to initiate the adaptive immune response
• Lymphatic system


  • Two plumbing systems: Circulatory system (under pressure) and Lymphatic system (not under pressure, drains fluid called lymph)
• Lymph


  1. Collected from the tissues into lymphatic vessels

  2. Passes through lymph nodes

  3. Transported in lymphatic vessels by muscular contractions & one way valve system

  4. Returns to blood
• Lymphoid organs and tissues

  • Primary lymphoid organs (bone marrow and thymus)
  • Secondary lymphoid organs (lymph nodes, spleen, MALT, GALT)
• Primary lymphoid organs


  Development and maturation of immune cells
• Secondary lymphoid organs


  Trap antigens, lymphocytes interact with antigens
• Thymus


  • Flat bi-lobed organ situated above the heart
  • Lobes separated into lobules
  • Outer cortex - packed with immature T cells (thymocytes)
  • Inner medulla - sparsely populated with thymocytes
  • Both criss-crossed with 3 dimensional stromal cell network composed of epithelial cells, dendritic cells and macrophages
• T cell development and maturation in thymus

  1. Positive selection of T cells with TCR which recognise self MHC molecules

  2. Elimination of T cells that cannot recognise antigen-MHC complexes, react too strongly with self MHC, or react too strongly with self MHC plus self peptides

  3. Over 95% immature T cells die by apoptosis
• Bone marrow


  Maturation, differentiation and selection of B cells, elimination of B cells with self-reactive antibody receptors
• Lymph nodes


  • Encapsulated bean shaped structures
  • Reticular network of lymphocytes, macrophages and dendritic cells
• Lymphoid follicles in secondary lymphoid organs

  1. Start as primary lymphoid follicles with follicular dendritic cells (FDC) rich in B cells

  2. FDC capture and display antigen to B cells

  3. B cells that recognise the antigen stay in the follicle and proliferate

  4. Secondary lymphoid follicle develops with germinal centre

  5. Selection process occurs, over 90% cells die by apoptosis, only B cells that bind strongly to the antigen survive and differentiate into plasma or memory cells
• Spleen


  • Major role is immune responses to antigens carried in the blood
  • Filters blood and traps blood borne antigens, responds to systemic infections
  • Red pulp - mainly macrophages and erythrocytes, site for destruction of old and defective erythrocytes
  • White pulp - periarteriolar lymphoid sheath (PALS) around arterioles with mainly T lymphocytes, marginal zone with mainly B cells, containing primary follicles and germinal follicles
• Secondary lymphoid tissues


  • Mucosal associated lymphoid tissue (MALT)
  • Peyers patches (small intestine)
  • Tonsils and adenoids
  • Appendix
  • Lymphoid follicles within the lamina propria of the intestines and in the mucus membranes lining the upper airways, bronchi and genital tract
• Hematopoiesis


  The process where all blood cells arise in the bone marrow from hematopoietic stem cells (HSC)
• Hematopoietic stem cells (HSC)


  • Self renewing by cell division
  • Scarce (<1 HSC per 5 x 10^4 cells)
  • Able to proliferate rapidly when necessary
• Hematopoiesis


  Early differentiation of HSC into common myeloid progenitor or common lymphoid progenitor cells
• Cells derived from common lymphoid progenitor

  • B lymphocytes
  • T lymphocytes
  • Natural killer cells
  • Dendritic cells
• Lymphocytes


  • T cells, B cells and natural killer cells (NK cells) derived from lymphoid progenitor cells
  • 20-40% of white blood cells, 99% of cells in lymph
  • Resting T and B cells are small, motile, non-phagocytic cells that cannot be distinguished morphologically
• Natural killer cells


  Play an important role in host defence against tumour cells and virus infected cells
• Dendritic cells


  • Arise from both myeloid and lymphoid progenitor cells
  • Long membrane extensions resembling dendrites of nerve cells
  • Potent antigen presenting cells, most process and present antigen to TH cells
• Cells derived from myeloid progenitor

  • Erythrocytes
  • Monocytes
  • Neutrophils
  • Eosinophils
  • Basophils
  • Megakaryocytes
  • Dendritic cells
• Mononuclear phagocytes


  Monocytes circulate in the blood, enlarge and migrate into the tissues to differentiate into specific tissue macrophages
• Types of macrophages


  • Alveolar macrophages (lung)
  • Histiocytes (connective tissue)
  • Kupffer cells (liver)
  • Mesangial cells (kidney)
  • Microglial cells (brain)
  • Osteoclasts (bone)
• Polymorphonuclear granulocytic cells


  • Neutrophils, eosinophils and basophils classified by cellular morphology and staining characteristics, derived from myeloid progenitor cells
• Neutrophils


  50-60% of white blood cells, circulate in the blood, highly phagocytic, contain chemicals to destroy invaders, exit blood and enter tissues in response to infection
• Eosinophils


  Motile phagocytic cells, can migrate from blood into tissues, thought to have a role in defence against parasitic organisms, eosinophil granules contain chemicals harmful to parasites
• Basophils


  Non-phagocytic granulocytes that circulate in the blood, granules release pharmacologically active substances, major role in allergic reactions