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PATHOGENS – assortment or viruses, bacteria, fungi and parasites. Each has a different lifestyle and attacks the body in a specific way.
LYMPHOID SYSTEM – includes the cells, tissues, and organs responsible for defending the body against both environmental hazards and internal threats.
Lymphocytes are the primary cells of this system. They act to eliminate or render them harmless through a combination of physical and chemical attacks.
IMMUNE RESPONSE – organization of lymphocytes to provide a specific defense.
IMMUNITY – ability to resist infection and disease through the activation of specific defenses.

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The lymphoid system contains:
1. LYMPH – a fluid that resembles plasma but contains much lower concentration of suspended protein.
2. LYMPHATIC VESSELS (LYMPHATICS) – begin in peripheral tissues and end at connections to veins.
3. LYMPHOIS TISSUES & LYMPHOID ORGANS – scattered throughout the body.
4. Lymphocytes & smaller phagocytes and other cells.
Functions of the Lymphoid System – production, maintenance, and distribution of lymphocytes that provide defense against infectious and other environmental hazards.
Lymphocytes made in tonsils, spleen, thymus and areas of red bone marrow.
LYMPHATIC CAPILLARIES – beginning of lymphatic network, branch through peripheral tissue. Lined by endothelial cells. Differ from blood capillaries by:
1. Originate as pockets rather than forming continuous tubes
2. Have larger diameters
3. Have thinner walls
4. Typically have a flattened or irregular outline in sectional view
Major Lymph-Collecting Vessels – two sets of vessels collect lymph:
SUPERFICIAL LYMPHATICS – located in subcutaneous layer of the skin…
DEEP LYYMPHATICS – are larger lymphatic vessels that accompany deep arteries and veins supplying skeletal muscles and other organs…
They converge to form larger vessels called the lymphatic trunk, which empty into the thoracic duct, body inferior of the diaphragm and from the left side of the body superior to the diaphragm. Also empties into right lymphatic duct which collects lymph from the right side of the body superior of the diaphragm.
LYMPHOCYTES – account for 20-30 % of circulating leucocyte population. Three types:
1. T – cells (80%)
2. B – cells
3. NK – cells
T cell types:
CYTOTOXIC T CELLS – attack foreign cells or body cells infected by viruses.
HELPER T CELLS – stimulate the activation and function of both T cells and B cells.
SUPPRESSER T CELLS – inhibit the activation and function of both T & B cells.
Lymphocyte Production
LYMPHOPOIESIS – lymphocyte production, involves the bone marrow, thymus, and peripheral lymphoid tissues.
B cells move into lymph nodes as they mature
NK migrate throughout the body in search of abnormal cells as they mature.
LYMPHOID TISSUES – connective tissues dominated by lymphocytes.
LYMPHOID NODULE – lymphocytes are densely packed in an area of areolar tissue, found in tonsils.
MALT (MUCOSA-ASSOCIATEDLYMPHOID TISSUE) – collection of lymphoid tissues linked with the digestive system.
TONSILS – large lymphoid nodules in the walls of the pharynx. Most people have five.
LYMPHOID ORGANS
LYMPH NODES – small lymphoid organs ranging in diameter from 1mm to an inch. Resembles kidney bean.
AFFERENT LYMPHATICS – carry lymph to the lymph node from peripheral tissues.
EFFERENT LYMPHATICS – leaves lymph node at the hilum, they carry lymph away from the node and towards venous circulation.
Lymph Node Function – functions like a kitchen water filter. They provide an early warning system
THYMUS – pink, grainy organ located in the mediastinum
SPLEEN – contains largest collection of lymphoid tissue in the body
Removes abnormal blood cells and other blood components by phagocytosis
Storage of iron recycled from RBCs
Initiation of immune responses
The lymphoid System and Body Defenses – body defenses can be sorted into two categories
1. Nonspecific defenses – does not distinguish one type of threat from another
2. Specific defenses – protect against