Immunology - Lymphoid Organs PDF

The Immune System and The Lymphoid Organs Neutralizes or inactivates foreign molecules (antigens) and destroys microorganisms or other cells, such as virus-infected cells, cells of transplanted organs, and cancer cells.  Types of Immune Responses 1. The innate response...

The Immune System and The Lymphoid Organs Neutralizes or inactivates foreign molecules (antigens) and destroys microorganisms or other cells, such as virus-infected cells, cells of transplanted organs, and cancer cells.  Types of Immune Responses 1. The innate response Is the first line of defense, fast and nonspecific. It does not produce memory cells. It consists of: 1. Physical barrier, such as the skin and mucous membrane 2. Chemical defenses, such as low pH 3. Various secretory substances, such as thiocyanate in saliva, lysozymes, interferons, fibronectin, and complement 4. Phagocytic cells, such as macrophages, neutrophils, and monocytes, and natural killer cells  Types of Immune Responses 2. The adaptive response Complex, slower and specific Depends on the initial recognition of antigens by lymphocytes Produces memory cells Types of adaptive immune response 1. Humoral immune response Via antibodies from plasma cells 2. Cellular immune responses Via T lymphocytes Secrete cytokines that act on other immune cells Attack foreign cells or cells that exhibit foreign epitopes on their surfaces  Antigens May be: Soluble molecules, such as: proteins, polysaccharides, and nucleoproteins, Molecules belonging to whole cells (bacteria, protozoa, tumour cells, or virus-infected cells). Have antigenic determinants or epitopes.  Antibodies Are glycoproteins and belong to the immunoglobulin (Ig) protein family, with several classes. Consist of two identical light chains and heavy chains, and two terminals: 1. Carboxyl-terminal (Fc region)  Some are bound to membrane of immune cells 2. Two antigen binding amino (NH2) ends – variable region  The main classes of immunoglobulins & their characteristics Types Structure % in Cells to which Functions (Isotyp serum binds via Fc es) of region Ig IgG Monomer 75- Macrophages, B Principal Ig in secondary 85% cells, NK cells, immune response neutrophils, Longest half life (23 days) eosinophils Activates complement Stimulates chemotaxis Crosses placental barrier IgA Dimer or 5– B cells Present in body secretions, trimer 15% such as nasal, bronchial, combined intestinal, and prostatic with secretion, as well as in secretory tears, colostrum, saliva, and componen vaginal fluid t Protects the surfaces of mucosa  The main classes of immunoglobulins & their characteristics IgM Pentamer 5-10% B cells First antibody to be produced in primary immune response. Activates complement. Activates macrophages. Serves as antigen receptor for B cells. IgD Monomer 0.2% B cells Serves as antigen receptor for B cells together with IgM. IgE Monomer 0.002% Mast cells Stimulates mast cells. and Is responsible for basophils anaphylactic hypersesitivity reactions. Increased levels in parasitic infections.  Actions of Antibodies 1. Agglutination and precipitation of soluble antigens. 2. Neutralization: block adhesion of microorganisms to cells and may also inactivate toxins  Actions of Antibodies 3. Binding of antibodies to the surface of worms activate immune cells bound to the antibody to induce lysis of the cell membrane of the worms.  Actions of Antibodies 4. By opsonisation of antigens stimulate phagocytosis. 5. Activates complement system which stimulates phagocytosis through opsonisation, and lysis of membranes of microorganisms.  Cells of the Immune System 1. Primarily lymphocytes – Able to selectively recognize a specific epitope among about 1018 different types of epitopes. 2. Various types of supporting cells that: – Include: monocytes, macrophages, neutrophils, basophils, eosinophils, reticular cells, dendritic cells, follicular dendritic cells, plasma cells, mast cells, and cells of the mononuclear phagocyte system. Specialized epithelial and stromal cells  B lymphocytes When stimulated proliferate and differentiate into plasma cells and B memory cells. – Require activation by T-helper lymphocytes  T lymphocytes Recognize by their T cell receptors, only complexes formed by small peptide epitopes and Major Histocompatibility Complex (MHC) exposed on the surface of other cells  Antigen-presenting cells (APC) Take up foreign particles, catabolise them and present antigens bound to their type II MHC to be recognized by CD4+ T (helper) cells. Include: 1. The mononucleated phagocytic system including macrophages, Kupffer cells of liver, Langerhans’ cells of epidermis, dendritic cells of spleen and lymph nodes 2. B lymphocytes 3. Type II and III epithelialreticular cells of thymus  Antigen-presenting cells (APC) All nucleated cells also able to present proteins from their own intracellular source bound as antigen to type I MHC. – If the proteins are derived from intracellular viruses or transformed cells, the cells will be destroyed by CD8+ T (cytotoxic) cells.  Lymphoid Tissue Are made up of cells with a reticular connective tissue stroma that are found: 1. Freely within the regular connective tissue or 2. Encapsulated, forming the lymphoid organs.  Primary (central) lymphoid organs Include: 1. Bone marrow and GALT (Bursa- equivalent organs) 2. Thymus Function in lymphocyte production and maturation. Relative prevalence of T and B lymphocytes in the major lymphoid organs Lymphoid T Lymphocytes, B Lymphocytes, Organ (%) (%) Thymus 100 0 Bone marrow 10 90 Spleen 45 55 Lymph nodes 60 40 Blood 75 25  Secondary lymphoid organs Include: 1. Spleen 2. Lymph node 3. Mucosa associated lymphoid tissue  Thymus Divides into incomplete lobules, each with cortex & medulla Grows until puberty (35 – 40 gm) & then begins to involute Contains  T Lymphocytes & macrophages  Epithelial reticular cells (ERC) – 3 each in cortex & medulla  Type VI makes Hassall’s corpuscles  T Lymphocyte maturation In the cortex types II & III ERC present self MHC and self & foreign antigens to the developing T lymphocytes, and those: – Positively reacting by recognizing these will be allowed to pass to medulla by positive selection – Do not recognize will be killed  T Lymphocyte maturation – In the medulla the T cells who have passed the positive selection in the cortex will be presented with self antigens displayed on self MHC, and those : Recognizing these will be killed by negative selection Do not recognize these will leave medulla to Thymic Dependent 20 Lymphoid Organs: Paracortex of lymph nodes, PALS of spleen & Peyer’s patches of ileum  Lymph Nodes Make a series of in-line lymph filters Divided into different regions: 1. Cortex a. Subcapsular lymphatic sinus b. Outer cortex c. Paracortex (inner cortex) with intermediate sinuses 2. Medulla a. Medullary lymphoid sinuses b. Medullary cords  Subcapsular lymphatic sinus Contains: 1. Dendritic cells 2. Macrophages 3. Lymphocytes 4. Reticular cells 5. Reticular fibres  Outer cortex of lymph node Contains:  A diffuse population of cells composed mainly of T lymphocytes, reticular cell, macrophages and antigen presenting dendritic cells  Lymphoid nodules as primary & secondary Paracortex (inner cortex) of lymph node Located between cortex & medulla Contains: Few nodules Mainly T lymphocytes Intermediate lymphatic sinuses  Medulla of Lymph Node Contains: 1. Medullary lymphoid sinuses 2. Medullary cords Contain: B lymphocytes, plasma cells, macrophages, reticular cells & fibres  Circulation of Lymph in the Lymph Node Afferent lymphatic vessels come via capsule and drains successively into:  Subcapsular sinus  Intermediate sinuses  Medullay sinuses  Efferent lymphatic vessels which contain newly formed & circulating lymphocytes More than 99% antigens & debris are removed  Circulation of Blood in the Lymph Nodes Small arteries come via hilum & medullary trabeculae to give: 1. Medullary & cortical capillary beds 2. Postcapillary venules as high endothelial venules (HEV) in the paracortex  Spleen Filters blood Capsule and trabeculae have a few smooth myocytes Is divided into: 1. White pulp (spot) 2. Red Pulp (spot)  White pulp (Malpighian bodies) Contain: 1. Periarterial lymphoid sheath (PALS) – mainly T lymphocytes 2. Lymphoid nodules – mainly B lymphocytes 3. Marginal zone  Marginal zone of white pulp Surround the white pulp Contains – Lymphoid tissue : B lymphocytes, plasma cells, macrophages & interdigitating dendritic cells (APC) – Marginal sinuses - show 2-3 µm wide space between endothelial cells for exit of blood born elements  Red pulp Contains: 1. Splenic sinuses (splenic sinuses of the red pulp ) 2. Splenic cords of Bilroth: macrophages, lymphocytes, plasma cells and many blood  Blood circulation in spleen Splenic arteries give successively Trabecular arteries Central (white pulp arteries) Penicillar arteries Three theories of splenic blood circulation 1. Closed circulation 2. Open circulation 3. Combination of both closed & open circulation types  Diffuse lymphoid system (unencapsulated lymphoid tissue) Called mucosa associated lymphoid tissue (MALT)  Include: 1. Gut associated lymphoid tissue (GALT) 2. Bronchus associated lymphoid tissue (BALT) 3. Tonsils a. Palatine tonsils b. Pharyngeal tonsil c. Lingual tonsil d. Tubal tonsils Gut associated lymphoid tissue (GALT) Found in the mucosa of the GIT Make Peyer’s patches in the ileum with:  B cells centrally, and T cells & APC peripherally  Microfold (M) cells replace lining epithelium  High endothelial venules  Palatine tonsils Have dense fibrous capsule on deeper aspect Superficial aspect with:  Stratified squamous nonkeratinized epithelium  10-20 crypts Contain many 10 & 20 lymphoid nodules  Pharyngeal tonsil Lined by pseudostratified ciliated epithelium with patches of stratified squamous epithelium Thin incomplete capsule on deeper aspect Show pleats with longitudinal folds & ducts of seromucous glands Called Adenoid when inflamed  Lingual tonsil Numerous on dorsal surface of posterior 3rd of tongue Superficially lined with stratified squamous nonkeratinized epithelium Have incomplete capsule on deeper aspect Each has a single crypt whose base receives ducts of seromucous glands  Tubal tonsils Around the opening of pharyngotympanic tube Not well developed