Lymphatic System

• The lymphatic system consists of lymph, lymphocytes, lymphatic vessels, lymph nodes, tonsils, the spleen, and the thymus gland.

Functions of the Lymphatic System

• The lymphatic system maintains fluid balance in tissues, absorbs fats from the small intestine, and defends against microorganisms and foreign substances.

Lymphatic Capillaries and Vessels

• Lymphatic vessels carry lymph away from tissues. Valves in the vessels ensure the one-way flow of lymph.

• Skeletal muscle contraction, contraction of lymphatic vessel smooth muscle, and thoracic pressure changes move the lymph through the vessels.

• The thoracic duct and right lymphatic duct empty lymph into the blood.

Lymphatic Organs

• Lymphatic tissue produces lymphocytes, when exposed to foreign substances, and it filters lymph and blood.

• The tonsils protect the openings between the nasal and oral cavities and the pharynx.

• Lymph nodes, located along lymphatic vessels, filter lymph.

• The white pulp of the spleen responds to foreign substances in the blood, whereas the red pulp phagocytizes foreign substances and worn out erythrocytes. The spleen also functions as a reservoir for blood.

• The thymus processes lymphocytes that move to other lymphatic tissue to respond to foreign substances.

Immunity

• Immunity is the ability to resist the harmful effects of microorganisms and other foreign substances.

Innate Immunity

Mechanical Mechanisms

• The skin and mucous membranes are barriers that prevent the entry of microorganisms into the body.

• Tears, saliva, and urine act to wash away microorganisms.

Chemical Mediators

• Chemical mediators kill microorganisms, promote phagocytosis, and increase inflammation.

• Lysozyme in tears and complement in plasma are examples of chemicals involved in innate immunity.

• Interferons prevent the replication of viruses.

Cells

• Chemotaxis is the ability of cells to move toward microorganisms or sites of tissue damage.

• Neutrophils are the first phagocytic cells to respond to microorganisms.

• Macrophages are large phagocytic cells that are active in
  the latter part of an infection. Macrophages are also
  positioned at sites of potential entry of microorganisms into tissues.

• Basophils and mast cells promote inflammation, whereas eosinophils inhibit inflammation.

• Natural killer cells lyse tumor cells and virus-infected cells.

Inflammatory Response

• Chemical mediators cause vasodilation and increase vascular permeability, allowing the entry of chemicals into damaged tissues. Chemicals also attract phagocytes.

• The amount of chemical mediators and phagocytes increases until the cause of the inflammation is destroyed. Then the tissues undergo repair.

• Local inflammation produces the symptoms of redness, heat, swelling, pain, and loss of function. Symptoms of systemic inflammation include an increase in neutrophil numbers, fever, and shock.

Adaptive Immunity

• Antigens are molecules that stimulate adaptive immunity.

• B cells are responsible for humoral, or antibody-mediated, immunity. T cells are involved with cell-mediated
  immunity.

Origin and Development of Lymphocytes

• B and T cells originate in red bone marrow. T cells are processed in the thymus and B cells are processed in red bone marrow.

• B and T cells move to lymphatic tissue from their processing sites. They continually circulate from one lymphatic tissue to another.

Activation and Multiplication of Lymphocytes

• B cells and T cells have antigen-binding receptors on their surfaces. Clones are lymphocytes with the same antigen-binding receptor.

• Major histocompatibility complex (MHC) molecules present processed antigens to B or T cells.

• Costimulation by cytokines, such as interleukins, and surface molecules, such as CD4, are required in addition to MHC molecules.

• Macrophages present processed antigens to helper T cells, which divide and increase in number.

• Helper T cells stimulate B cells to divide and differentiate into cells that produce antibodies.

Antibody-Mediated Immunity

• Antibodies are proteins. The variable region combines with antigens and is responsible for antibody specificity. The constant region activates complement or attaches the antibody to cells. The five classes of antibodies are IgG, IgM, IgA, IgE, and IgD.

• Antibodies directly inactivate antigens or cause them to clump together. Antibodies indirectly destroy antigens by promoting phagocytosis and inflammation.

• The primary response results from the first exposure to an antigen. B cells form plasma cells, which produce antibodies, and memory B cells.

• The secondary (memory) response results from exposure to an antigen after a primary response. Memory B cells quickly form plasma cells and new memory B cells.

Cell-Mediated Immunity

• Exposure to an antigen activates cytotoxic T cells and produces memory T cells.

• Cytotoxic T cells lyse virus-infected cells, tumor cells, and tissue transplants. Cytotoxic T cells produce cytokines, which promote inflammation and phagocytosis.

Immune Interactions

Innate immunity, antibody-mediated immunity, and cell-mediated immunity can function together to eliminate an antigen.

Immunotherapy

Immunotherapy stimulates or inhibits the immune system to treat diseases.

Acquired Immunity

• Active natural immunity results from everyday exposure to an antigen against which the person's own immune system mounts a response.

• Active artificial immunity results from deliberate exposure to an antigen (vaccine) to which the person's own immune system responds.

• Passive natural immunity is the transfer of antibodies from a mother to her fetus during gestation or baby during breastfeeding.

• Passive artificial immunity is the transfer of antibodies from an animal or another person to a person requiring immunity.