Chapter 9
Immunity and the Lymphatic System
Our body’s innate (non-specific) defenses
· First line of defense
o Barriers to entry - both physical and chemical
· Second line of defense
o Phagocytic white blood cells
o Inflammatory response
o Protective proteins - complement and interferons
o Fever
· The first line of defense
o Physical barriers
o Skin
o Tears, saliva, and urine physically flush out microbes
o Mucous membranes line the respiratory, digestive, reproductive, and urinary tracts
o Resident bacteria/normal flora that inhabit the body use available nutrients and space thus preventing pathogens from taking up residence
o Protective proteins
§ Complement
· A series of chemical reactions leads to the complement cascade - various proteins aggregate in a manner that punches holes in bacterial cell membrane
§ Interferon
· Signal the progression of a viral infection to adjacent cells inhibiting the spread of the infection
· The second line of defense: Phagocytic white blood cells
o Includes neutrophils and macrophages
§ Both leave the circulation and move into the appropriate tissue
§ Important in the inflammatory response
· The second line of defense: Inflammatory response
o Four hallmark symptoms are redness, heat, swelling and pain
o Histamine released by mast cells causes the capillaries to dilate and become more permeable including to phagocytic white blood cells
o Increased blood flow to an area increases the warmth that inhibits some pathogens
o Increased blood flow also brings more white blood cells to an injured area with neutrophils being the first scouts to kill the pathogens
o This response can be short-lived but if the neutrophils cannot control the damage, cytokines (chemicals) will recruit more white blood cells, including macrophages
Functions of the lymphatic system
§ Lymphatic capillaries absorb excess tissue fluid and return it to the bloodstream
§ Lymphatic capillaries (lacteals) in the small intestine absorb fats and transport them to the bloodstream
§ Works in the production, maintenance, and distribution of lymphocytes in the body
o Helps in defense against pathogens
§ Lymphatic vessels
o One-way valve system that carries fluid called lymph
o Made of capillaries, vessels and ducts
o Function to return tissue fluid (including water, solutes, and cell products) to the bloodstream
o The larger vessels are similar in structure to veins and have valves
§ Lymphatic organs
o Responsible for filtering, cleaning, and returning lymph to the bloodstream
o Red bone marrow
o Thymus gland
o Lymph nodes and spleen
o Tonsils
The specific defenses of the immune system
Third line of defense
o Helps protect us against specific pathogens when nonspecific defenses are insufficient
o Helps protect us against cancer
o Depends on the action of B and T cells (lymphocytes)
o B cells
o Mature in the bone marrow
o Usually found inside lymph nodes and interstitial fluid
o Produce antibodies specific for a particular pathogen
o T cells
o Mature in the thymus
o Do not produce antibodies but do have specific receptors
o Important for stimulating B cells in response to pathogen attacks and directly killing infected cells
Antibody-mediated immunity by B cells
o Each B cell has a unique receptor called a BCR that binds a specific antigen
o This binding and cytokines secreted by helper T cells result in clonal expansion of only the activated B cells
o Most of the cells produced via clonal expansion are plasma cells that secrete antibodies
o Other cells become memory cells which result in long-term immunity
o After an infection has passed plasma cells undergo apoptosis (programmed cell death) leaving memory cells
Structure of antibodies
o A Y-shaped protein
o The trunk of the Y is a constant region that determines the class of the antibody
o The end of the arms (Y) are the variable regions where specific antigens bind
o IgG - occurs in circulating blood, lymph, and interstitial fluid
o IgM - first antibody produced in response to a pathogen attack and predominant antibody produced in infants
o IgA - found in secretions such as saliva and breast milk
o IgD - present on the surface of mature B cells, responsible for binding of antigens to activate the B cells
o IgE - found on the surface of basophils and mast cells and involved in immediate allergic reactions
Cell-mediated immunity by T cells
o Each T cell has a unique receptor called a TCR that will recognize a piece of an antigen with the help of an antigen-presenting cell (APC)
o An APC engulfs an antigen, breaks it down and presents it on its surface in association with a membrane protein called an MHC [called human leukocyte antigens (HLA)] to T cells in the lymph node or spleen
o The T cell will specifically recognize the combination of the HLA protein and the piece of antigen
o Clonal expansion will occur leading to mostly helper T cells, cytotoxic T cells, and a few memory T cells
o After an infection has passed, helper and cytotoxic T cells undergo apoptosis leaving memory cells
Helper T cells
o Secrete cytokines that help many immune cells function
Cytotoxic T cells
o Have vacuoles containing perforins and destructive enzymes
o Perforins punch holes in target cells followed by enzymes that cause the cell to undergo apoptosis
Natural killer cells
Killer cells
o Some T cells differentiate into natural killer cells
o These serve as a surveillance system against cancer cells and viral infected cells
o Separate from the specific immune response because they are not specific
Active and passive immunity
o The ability to combat diseases and cancer
o Can be brought about naturally through an infection or artificially through medical intervention
There are two types of immunity - active and passive
Active immunity
o The individual’s body makes antibodies against a particular antigen
o This can happen through natural infection or through immunization using vaccines
o The primary response is slower and shorter-lived while the secondary exposure is a rapid, strong response
o This type of immunity is usually long-lasting
o It depends on memory B and T cells
Passive immunity
o An individual is given prepared antibodies against a particular antigen
o This type of immunity is short-lived
o This can happen naturally as antibodies are passed from mother to fetus or artificially via an injection of antibodies
How can the immune system react in ways that may be harmful to the body?
o Allergies
o Tissue rejection
o Immune system disorders
Disorders of the immune system
o Autoimmune diseases:
o A disease in which cytotoxic T cells or antibodies attack the body’s own cells as if they were foreign
o Examples include multiple sclerosis, lupus, myasthenia gravis, and rheumatoid arthritis
o Immunodeficiency disease:
o A disease in which the immune system is compromised and thus unable to defend the body against disease
o Examples include AIDS and SCID