Lymphoid

• Immunity


  All those physiological mechanisms that endow the animal with the capacity to recognize materials as foreign to itself and to neutralize, eliminate or metabolize them with or without injury to its own tissues
• Immune system


  • Structured to recognize, respond to, and destroy a wide variety of invading organisms that would otherwise be capable of promoting infections harmful to the body
• Lymphocytes

  • Represent 20-40% of the circulating WBCs
  • Arise from a hematopoietic stem cell and then are further differentiated in the primary lymphoid organs
  • Can be separated into two main classes, depending on where this differentiation takes place
  • Circulation is complex and is regulated by different cell surface adhesion molecules and by chemical messengers called cytokines
• Primary Lymphoid Organs

  • Bone marrow
  • Thymus
• Secondary Lymphoid Organs

  • Spleen
  • Lymph nodes
  • Mucosal-Associated Lymphoid Tissue
  • Appendix
  • Tonsils
  • Peyer’s Patches
  • Cutaneous-Associated Lymphoid Tissue
• Bone Marrow


  • Main source of hematopoietic stem cells, which develop into different cell types
  • Lymphocyte stem cells are released from the marrow and travel to additional primary lymphoid organs where further maturation takes place
• Thymus

  • Small, flat, bilobed organ found in the thorax, right below the thyroid gland and overlying the heart
  • Each lobe is divided into lobules filled with epithelial cells that play a central role in the differentiation process
  • Mature T lymphocytes are released from the medulla
• Spleen

  • Largest secondary lymphoid organ
  • Located in the upper-left quadrant of the abdomen, just below the diaphragm
  • Splenic tissue can be divided into Red Pulp and White Pulp
• Lymph Nodes


  • Located along lymphatic ducts and serve as central collecting points for lymph fluid
• Lymphoid tissue contains a periarteriolar lymphoid sheath arranged around arterioles
• Primary follicles are attached to the sheath
• The marginal zone surrounding the PALS contains dendritic cells that trap antigen
• Lymph nodes are located along lymphatic ducts and serve as central collecting points for lymph fluid from adjacent tissues
• Lymph nodes are especially numerous near joints and where the arms and legs join the body
• Lymph fluid flows slowly through sinuses lined with macrophages in lymph nodes, creating an ideal location for phagocytosis
• Regions of Lymph Nodes

  • Cortex
  • Paracortex
  • Medulla
• Cortex of Lymph Nodes

  • Contains macrophages, aggregations of B cells in primary follicles, follicular dendritic cells, secondary follicles (germinal center), plasma cells, and memory cells
• Paracortex of Lymph Nodes

  • T-cells
• Medulla of Lymph Nodes

  • Some T cells, B cells, and numerous plasma cells
• Within secondary organs, T and B cells are segregated and perform specialized functions
• B cells differentiate into memory cells and plasma cells and are responsible for humoral immunity
• T cells play a role in cell-mediated immunity and produce sensitized lymphocytes that secrete cytokines
• Cytokines: '“Cytokines are small polypeptides that regulate the functions of lymphocytes and other cells involved in the immune response”'
• Surface Markers on Lymphocytes

  • Proteins that appear on cell surfaces can be used as markers to differentiate T cells and B cells
  • Antigens detected by monoclonal antibodies
  • Panels of antibodies from different laboratories used for analysis
  • Antibodies reacting similarly with standard cell lines define Cluster of Differentiation
• In blood circulation, T-cells and B-cells are present
• 1. cells differentiate into memory cells and plasma cells and are responsible for humoral immunity or antibody formation
• Pro-B cells rearrange genes that code for heavy and light chains of an antibody molecule
• Pre-B cells synthesize μ chains, lose the CD43 marker, c-Kit, and TdT
• Pre-B cells may express μ chains on the cell surface accompanied by a surrogate light chain
• Pre-B cells expressing μ heavy chains in association with surrogate light chains survive and proceed to further differentiation
• Once the pre-B receptor (pre-BCR) is expressed, neighboring pre-
• Formation of pre-B cell receptor

  Surrogate light chains plus two very short chains, Ig-α/Ig-β form the pre-B cell receptor
• Only pre-B cells expressing μ heavy chains in association with surrogate light chains survive


  Proceed to further differentiation
• Once pre-B receptor (pre-BCR) is expressed


  Neighboring pre-B cells may send signals for further maturation
• Rearrangement of genetic sequence coding for light chains

  On chromosome 2 or 22
• Completion of light chain rearrangement

  Commits a cell to produce an antibody molecule with specificity for a particular antigen or group of related antigens
• Once surface immunoglobulins appear

  μ chains are no longer detectable in the cytoplasm
• Surface proteins on immature B cells

  • CD21
  • CD 40
  • Major histocompatibility complex (MHC) class II molecules
• Immature B cells leave the bone marrow


  Proceed to seed the spleen and other secondary lymphoid organs
• Development of immature B cells into mature cells in the spleen

  Marginal zone B cells