Lymphocytes and Immune Response

• Lymphocytes elicit an immune response to foreign material
• Lymphocytes are a type of white blood cell
• Lymphocytes develop from lymphoblasts within lymphoid tissue, for eg. thymus

Types of Lymphocytes:

• Natural killer cells (NK)
• B lymphocytes
• T lymphocytes

Abnormal Lymphocyte Levels

• Some mutations in tumour suppression pathways can cause uncontrolled growth of B and T lymphocytes
• HIV decreases the number and function of T lymphocytes, cannot fight infection
• T cells initiate damage to the nervous system in multiple sclerosis

Lymphoid Cells

• Formed and matured in bone marrow
• Respond to pathogens by producing and dispatching antigen-specific antibodies, that neutralize foreign objects
• Found in the Lymph Node cortex dividing B cells, in germinal centers

CD8 T CELL

• Targets pathogens associated with an antigen

T Cells

• Some T-Cells are formed in the bone marrow, and mature in the thymus
• Some T-Cells are involved in cell-to-cell (cell mediated) immunity
• T-Cells are located in the deep cortex
• Two types are Helper T cells (CD4) and Cytotoxic T cells (CD8)

Primary Lymphoid Organs

• Includes red bone marrow, where haemopoiesis which produces all formed elements of the blood
• Bone marrow is diffusely located in central portions of flat bones and epiphyses of long bones
• B lymphocyte production and maturation occurs within the bone marrow

Thymus

• Composed of two identical lobes, located in the superior and anterior mediastinum
• It enlarges during childhood, atrophies at puberty, and is generally replaced with fat
• Main components are the cortex and medulla
• T-cells are generated in the bone marrow and travel to the thymus to mature
• Lymphoid stem cells in the cortex divide, mature into T cells and migrate into the medulla
• T cells that attack self-antigens are destroyed by the thymus during selection processes in lymphocyte proliferation and maturation

Secondary Lymphoid Organs

• Lymph nodes filter lymph
• They trap foreign material such as bacterial and viral particles
• The largest Lymphoid organ is the Spleen: Located in the left upper quadrant, between the 9th and 11th ribs
• The spleen removes abnormal blood cells and other blood components through phagocytosis
• Initiates an immune response by B and T cells following the stimulation of circulating antigens in the blood
• Note that spleens filter blood, whilst lymph nodes filter lymph

Lymphatic System

• Weekly learning objective is to describe the structure and function of the lymphatic system summarize the clinically relevant location of node groups
• The student should demonstrate an understanding of the key functions of the lymphatic system
• The student should demonstrate an understanding of the structure, development, and location of lymphoid tissue (primary vs secondary organs)
• Lymphatic transport and distribution is also important to be able to discuss
• The clinical implications of differing lymphoid system disorders should be considered

Lymphatic System Function

• Returns fluids that have leaked from the blood vascular system back to blood (15% of the bodies fluid)
• It protects the body by removing foreign materials like pathogens
• Transports large molecules that cannot diffuse through the capillary wall to enter blood

Interstitial Space and Fluid (IF)

• Approximately 85% of the fluid lost at the arterial end of the capillary bed is reclaimed into the blood at the venous end
• Waste products like O2, water, CO2, and nitrogenous waste are components
• Other components include electrolytes, amino acids, hormones, and lipids

Fluid Movement

• Approximately 20L seeps out, 17L reabsorbed, 3L left in tissues
• Interstitial fluid (IF) enters lymphatic capillary
• Lymph then flows to larger lymphatic vessels, both superficial and deep
• Lymphatic vessels then lead to lymphatic trunks
• Trunks then unite and either form the right lymphatic trunk and the thoracic duct

Lymphatic Vessels

• Two sets of vessels drain lymph from the capillaries including a superficial and deep lymphatic vessel
• Superficial vessels travel with superficial veins, are located in the subcutaneous layer next to the skin
• Superficial vessels are located in the loose connective tissue of the mucous membrane lining the digestive, respiratory, urinary and reproductive tracts
• They also occur in the loose connective tissues of the serous membranes lining the pleural, pericardial and peritoneal cavities
• Deep vessels accompany large arteries and veins; and they drain the skeletal muscle as well as organs in the neck, limbs and trunk
• They also drain visceral organs in the thoracic and abdominopelvic cavities.

Lymph Nodes

• Lymph enters the node via afferent vessels
• Dendritic cells present pathogens to T-helper cells
• Lymph moves through the subscapular sinus and into smaller sinuses
• Lymph moves through the intermediate sinus to the medullary sinus
• Lymph exits the node at its hilum via efferent vessels

Lymphatic Vessels

• There as two sets of vessels that drain lymph from the capillaries including a superficial and deep lymphatic vessel
• Superficial vessels travel with superficial veins and are located in the subcutaneous layer next to the skin
• Superficial vessels are located in the loose connective tissue of the mucous membrane lining the digestive, respiratory, urinary and reproductive tracts
• Deep vessels accompany large arteries and veins; and drain the skeletal muscle as well as organs in the neck, limbs and trunk
• These vessels drain visceral organs in the thoracic and abdominopelvic cavities

Lymphatic Trunks

• Lumbar trunks drain the legs, pelvic region, and kidneys.
• The intestinal trunk receives chyle (lymph mixed with fats) from the intestines.
• Bronchomediastinal trunks are located in the chest, they drain lymph from the lungs, heart, trachea, mediastinal, and mammary glands.
• Subclavian trunks are located beneath the clavicle, as they drain lymph fluid from the apical lymph nodes around the armpit, which carry lymph from the arms
• Jugular trunks are located in the neck; they drain lymph fluid from the cervical lymph nodes of the neck.

Lymph Ducts

• Receive lymph from the lymph trunks, and are the largest lymph vessels
• Vessels contain three layers, similar to those of great veins
• The thoracic duct receives from both sides of the body inferior to the diaphragm and left side of head and neck, thorax, and upper limb
• The right lymphatic duct drains the right upper quadrant above the diaphragm (head/neck, thorax, arm)
• Both ducts drain lymph from the venous angle between the internal jugular and subclavian veins, and enter the heart via the superior vena cava

Mucosa Associated Lymphoid Tissue (MALT)

• MALT are strategically located in mucous membranes and are the most extensive component of human lymphoid tissue
• First line of protection as they can be classified as lymphoid nodules, and do not have a capsule with a germinal center with activated lymphocytes
• MALT includes tonsils, Peyer’s patches and the appendix

Tonsils:

• Form a ring of respiratory defence around the entrance of the pharynx (throat) and appear as swellings in the mucosa
• The pharyngeal tonsil is located in the posterior wall of the nasopharynx, where, when enlarged they are known as adenoids
• Palatine tonsils (paired) are found on either side at the posterior end of the oral cavity (most often infected)
• Tubal tonsils surround the opening of the auditory tubes in the pharynx
• Lingual tonsils are found at the base of the tongue