Exam 2 Review:  Chapter 20:  Histology of the Lymph Node

capsule - A fibrous, membranous, or fatty sheath that encloses an organ or body part; the capsule of a lymph node is a thin layer of dense fibrous connective tissue penetrated by various afferent and efferent lymphatic vessels; the capsule of the lymph node separates the inner lymphoid tissue from the surrounding connective tissues and serves as a limiting barrier to prevent diffusion of lymph into the surrounding tissues; it may also serve as a barrier to the spread of infections or cancers that reach the lymph node.

cortex - The outer layer of an internal organ or body structure, e.g., the cerebrum, kidney, adrenal gland, or lymph node; the cortex of a lymph node contains many lymph follicles (dendritic cells surrounding germinal centers) which are separated by connective tissue trabeculae and the cortex contains a branching system of channels which route lymph from the afferent lymphatic vessels to the medulla; antigen-presenting macrophages and T and B lymphocytes in the cortex are involved in immune reactions to specific antigens.

medulla - The inner core of an internal organ or body structure, e.g., the cerebrum, kidney, adrenal gland, or lymph node; the medulla of a lymph node contains medullary cords (thin inward extensions of cortical lymphoid tissue) and lymph sinuses spanned by crisscrossing reticular fibers which act as a filter for the passage of lymph from the cortex to the efferent lymphatic vessels which carry lymph away from the lymph node at the hilus; the medulla also contains many macrophages.

trabeculae - (1) Any of the supporting strands of fibrous connective tissue projecting into an organ and constituting part of the framework of that organ. (2) Any of the fine spicules forming a network in spongy = cancellous bone.

lobule - Any small lobe or subdivision of a lobe of an organ, usually separated by a layer of fibrous connective tissue; the cortex of a lymph node contains lobules which house lymph follicles (dendritic cells surrounding germinal centers) which are separated by connective tissue trabeculae.

follicles - A general anatomical term describing small, often microscopic,  structures which tend to be rounded and often consist of concentric layers of cells and sometimes with a central fluid filled cavity; the follicles of a lymph node contain dendritic cells surrounding germinal centers containing antigen-presenting macrophages and T and B lymphocytes involved in immune reactions to specific antigens.

germinal center - The light-staining interior of a lymph follicle which contains a few dendritic cells, some antigen-presenting macrophages and many activated proliferating T and B lymphocytes, particularly B lymphocytes, which are involved in immune reactions to specific antigens.

medullary cord - A thin inward extension of cortical lymphoid tissue (T and B lymphocytes) which is bounded by lymph sinuses in the inner core (medulla) of a lymph node

B lymphocyte = B cell - A functional subclass of  lymphocytes (the smaller of the two kinds of agranular leukocytes), originating and maturing in the bone marrow; identified by the specific cell surface markers they express; they function in humoral immunity = antibody-mediated immunity (AMI); they make antibodies = immunoglobulins in a complex process requiring lymphokine stimulation and reaction with a specific antigens, and usually the participation of antigen-presenting macrophages and T lymphocytes = T cells; when actively involved in immune defense they may transform into plasma cells; any one B lymphocyte makes only one specific antibody.

T lymphocyte = T cell - A functional subclass of  lymphocytes (the smaller of the two kinds of agranular leukocytes), originating in the bone marrow and and maturing in the thymus; identified by the specific cell surface markers they express; (they do not make antibodies = immunoglobulins); they function in cellular immunity = cell-mediated immunity (CMI); they orchestrates and regulate many of the immune system's responses; several functional subcategories are recognized including memory T cells, helper Th cells, suppressor Ts cells, cytotoxic Tc cells, and delayed hypersensitivity Tdh cells.

macrophage - Any of the large long-lived phagocytic cells derived from monocytes comprising a major component of the diffuse immune system (sometimes called the reticuloendothelial system or, more recently, the mononuclear phagocytic system); these phagocytes from different sites in different organs and tissues have somewhat different properties; in addition to contributing to the killing of microorganisms and tumor cells; they release immune regulatory substances and play a vital role in antigen-presentation leading to the production of antibodies by B lymphocytes and the activation of T lymphocytes.

afferent lymphatic vessel - Any of the delicate thin-walled vascular tubes lined with endothelium and with valves which transport lymph (interstitial fluid = intercellular fluid = tissue fluid) from the tissue spaces into the interior of the cortex of a lymph node by passing the fluid through an opening in the capsule; lymph flow is regulated by Interstitial Fluid Hydrostatic Pressure, the muscular pump in the limbs, the thoracic pump in the chest, and valves which prevent backflow.

efferent lymphatic vessel - Any of the delicate thin-walled vascular tubes lined with endothelium and with valves which transport lymph (interstitial fluid = intercellular fluid = tissue fluid) from the interior (medulla) of a lymph node by exiting through an opening in the hilus and on through the lymphatic circulation; lymph flow is regulated by Interstitial Fluid Hydrostatic Pressure, the muscular pump in the limbs, the thoracic pump in the chest, and valves which prevent backflow.

sinuses - (1) A dilated channel or receptacle containing a fluid; e.g., venous blood or lymph; the sinuses of the lymph node are located in the cortex and the medulla and are separated by the trabeculae and medullary cords respectively.  (2) Any of various air-filled cavities in the bones of the skull communicating with the nostrils.

hilus - A depression or fissure where vessels or nerves or ducts enter or exit an organ, e.g., the kidneys, lungs, and lymph nodes; the hilus of the lymph node is where the efferent lymphatic vessels carry lymph away from the medulla lymph node and on in the lymphatic circulation.

Sketch and label:

2. The structure of a lymph node. What kinds of cells are common in lymph nodes and what are their functions.

 
fibroblast formation and maintenance of ground substance and protein fibers (collagen and reticular fibers) of fibrous capsule, trabeculae, and stroma.
erythrocyte delivers oxygen to other lymph node cell types
monocyte-derived macrophage phagocytosis of cellular debris, micro-organisms, tumor cells; antigen-presentation to T and B lymphocytes
B lymphocyte/plasma cell upon antigen-presentation, certain lineages of B lymphocytes will become activated to produce clones of antibody-producing plasma cells and smaller numbers of B memory cells
T lymphocyte (includes Tc,  Th, and  Ts subtypes) upon antigen-presentation, certain lineages of Tcytotoxic = Tc lymphocytes will become activated to produce clones of agressive immunodefensive cells and smaller numbers of Tc memory cells; Thelper =Th, and  Tsupressor = Ts subtypes will be involved in positive and negative feedback control of various immune response including Tc and B lymphocyte activation respectively
granulocyte (neutrophil, eosinophil, basophil) very minor component of lymph node cell population, performing their normal immune defensive functions
[Note:  T lymphocytes and macrophages are more common in the outer layer of lymph follicles while B lymphocytes and plasma cells are more common in the inner germinal center of lymph follicles.]