the lymphatic system ch 7

**The lymphatic system.**

•           Consists of **lymphatic vessels** and **organs.**

•           Four main functions:

•           Lymphatic capillaries absorb excess **interstitial** **fluid** and return it to the bloodstream.

•           Lymphatic capillaries called **lacteals** in the small intestine absorb dietary fats and transport them to the bloodstream.

•           Production, maintenance, and distribution of lymphocytes in the body.

•           Helps in defense against pathogens.

**Lymphatic vessels.**

•           Carry a fluid called **lymph.**

•           Begin as blind-ended **lymphatic capillaries** in the tissues.

•           The anatomy of the larger lymphatic vessels is similar to that of cardiovascular veins, including the presence of valves.

•           Valves prevent lymph from flowing backward.

•           The movement of lymph is largely dependent on skeletal muscle contraction.

**Lymphatic organs.**

•           Two types: **primary** and **secondary.**

•           **Primary lymphatic organs:** red bone marrow, thymus.

•           **Secondary lymphatic organs:** lymph nodes, spleen, tonsils.

**The Primary Lymphatic Organs**

**Red bone marrow.**

•           The site of blood cell production.

•           In children, most bones have red marrow; only a few do in adults.

•           **B cells (B lymphocytes)** mature here.

**Thymus.**

•           Bilobed; in the thoracic cavity superior to the heart.

•           It is largest in children and shrinks as we age.

•           Maturation of T cells

**The Secondary Lymphatic Organs**

**Spleen.**

•           Filters blood.

•           In the upper left region of the abdominal cavity.

•           Has a thin outer capsule, so can rupture from trauma.

**Lymph nodes.**

•           Found along the lymphatic vessels.

•           Filter lymph.

•           Connective tissue forms a capsule around it and divides it into compartments.

•           Filled with macrophages that engulf pathogens and debris.

•           Also houses lymphocytes, which fight infections and cancer cells.

•           Named for their location: common in the neck, armpit, and groin.

**Lymphatic nodules.**

•           Concentrations of lymphoid tissue that don’t have a capsule.

**Tonsils** are located in the pharynx.

•           Have the same function as lymph nodes; fight infections that come in through the nose and mouth.

**Peyer patches.**

•           Found in the intestinal walls and the appendix.

•           Fight infections that come in via the digestive tract.

**Immunity**—killing or removing foreign substances, pathogens, and cancer cells from the body.

There are two branches of our immune system: **innate** and **adaptive.**

•           Innate—fully functional without previous exposure to a pathogen.

•           Adaptive—is initiated when exposed to a pathogen.

**Innate (nonspecific) immune defenses** include:

•           **Physical** and **chemical barriers.**

•           **The inflammatory response.**

•           **Protective proteins.**

Innate defenses have no recognition of a pathogen, and no memory**.**

**Physical and Chemical Barriers**

First line of defense against pathogens.

•           Barriers to entry: physical and chemical.

•           Physical: skin and mucous membranes.

•           The skin has lots of keratin, so is tough; also, exfoliation carries microbes away from the body.

•           Mucous membranes have ciliated cells; the cilia sweep away mucus with entrapped pathogens.

•           Chemical:

•           Acidic secretions of sebaceous glands weaken, kill bacteria on the skin.

•           Sweat, saliva, and tears have **lysozyme**—an antibacterial enzyme.

•           Chemical barriers, continued.

•           The acidic pH of the stomach inhibits or kills bacteria.

•           Normal flora (microbes that usually reside in the mouth and intestine) prevent potential pathogens from taking up residence.

•           Chronic use of antibiotics can make one susceptible to infection by killing off the normal flora.

**Inflammatory response.**

•           Second line of defense against pathogens.

•           Employs mainly neutrophils and macrophages to engulf pathogens.

•           Four hallmark symptoms: **redness, heat, swelling,** and **pain.**

•           Chemicals such as **histamine**, released by damaged tissue cells and mast cells, cause the capillaries to dilate and become more permeable.

•           Four hallmark symptoms: **redness, heat, swelling,** and **pain,** continued.

•           Excess blood flow causes the skin to redden and become warm.

•           Increased temperature inhibits growth of pathogens.

•           Increased blood flow brings white blood cells to the area.

•           Increased permeability of capillaries allows fluids and proteins to escape into the tissues.

•           Blood clot prevents blood loss.

•           Excess fluid in the area (swelling) presses on nerve endings, causing pain.

**Inflammatory response**

•           WBCs move out of the bloodstream into the surrounding tissue.

•           Neutrophils are first; they phagocytize debris, dead cells, and bacteria.

•           Can usually localize any infection and keep it from spreading.

•           If die off in great quantities, they become a yellow-white substance called pus.

•           If neutrophils become overwhelmed, they secrete **cytokines**—chemicals that attract more WBCs.

•           Monocytes that come to the area become **macrophages**—powerful phagocytes.

**Protective proteins.**

•           **Complement system**—several plasma proteins designated by the letter C and a number.

•           Are involved in and amplify the inflammatory response.

•           Some bind to mast cells, triggering histamine release.

•           Others attract phagocytes to the scene.

•           Some form a membrane attack complex that produces holes in the surface of bacteria; fluids enter the bacterial cells and they burst.

•           **Interferons.**

•           Chemicals produced by cells that are infected with viruses as a warning to other cells.

•           Bind to receptors of noninfected cells, causing them to produce substances that interfere with viral replication.

**Adaptive defenses**

•           Come into play when innate (nonspecific) defenses have failed to prevent an infection.

•           Provide some protection against cancer.

•           Respond to **antigens** (immune system recognizes as foreign).

•           Fragments of bacteria, viruses, molds, and parasitic worms can all be antigenic.

•           Abnormal plasma membrane proteins produced by cancer cells may also be antigens.

•           The immune system is able to distinguish ‘self’ (our cells) from ‘nonself’ (pathogens).

**How Adaptive Defenses Work**

Depend on the action of B cells (B lymphocytes) or T cells (T lymphocytes).

•           B cells and T cells have specific antigen receptors (plasma membrane proteins that bind to particular antigens).

•           Each lymphocyte has only one type of receptor.

•           The receptor and antigen fit together like a lock and key.

There are two pathways of adaptive immunity: **cell-mediated** and **antibody-mediated.**

•           In cell-mediated immunity, T cells kill cells that are presenting a specific “foreign” antigen.

•           In antibody-mediated immunity (also called **humoral immunity**), B cells produce antibodies that bind to free antigens in body fluids.

**T Cells and Cell-Mediated Immunity**

**T-cell receptor (TCR)**—unique receptor on the surface of each T cell.

•           B cells also have unique receptors, but unlike B cells, T cells are unable to recognize an antigen without help; it must be displayed to them by an **antigen-presenting cell (APC)** such as a macrophage.

•           After phagocytizing a pathogen, APCs travel to the

T cells in a lymph node or the spleen.

•           APCs break the ingested pathogen apart in a lysosome.

•           A piece of the pathogen is then displayed in the groove of a **major histocompatibility complex (MHC)** protein on the cell’s surface. The two classes of MHC proteins are called **MHC I** and **MHC II.**

•           Human MHC II proteins are called **human leukocyte antigens (HLAs).**

•           Found on all body cells.

•           There are three general groups of HLAs **(HLA-A, HLA-B, and HLA-DR)**, each with a number of protein variations.

•           Each person has a unique combination of HLAs.

•           HLAs of identical twins, however, are identical.

**Clonal Expansion.**

•           Many copies of the activated T cell are produced.

•           Occurs when a macrophage presents an antigen to a T cell that has the specific TCR that will bind this particular antigen.

•           This activates the T cell, causing it to undergo clonal expansion.

•           Some T cells become **cytotoxic T cells**, and some will become **helper T cells.**

**Cytotoxic T cell.**

•           Has storage vacuoles that contain perforins

•           After binding to a virus-infected cell or tumor cell, it releases perforins, which punch holes in the plasma membrane, forming a pore.

•           Responsible for cell-mediated immunity.

**Helper T cells.**

•           Secrete cytokines that enhance the response of all types of immune cells.

•           B cells cannot be activated without T-cell help.

•           The human immunodeficiency virus (HIV), which causes AIDS, infects helper T cells, thus inactivating the immune response.

**B Cells and Antibody-Mediated Immunity**

**B-cell receptors (BCR)**—protein receptors on B cells.

•           An antigen binds to the BCR on only one type of B cell.

•           That one B cell then produces copies of itself—this group of identical B cells is called a **clone.**

**B Cells Become Plasma Cells and Memory B Cells**

During clonal expansion, cytokines secreted by helper T (TH) cells stimulate B cells to clone

•           Most of the cloned B cells become **plasma cells**, which produce large numbers of antibodies that bind to the antigen that initiated this whole process.

•           Some B cells become **memory cells**, which become active in future encounters of this same antigen

•           Confer immunity to that antigen.

**Structure of an antibody.**

•           Y-shaped protein with two arms.

•           Each arm has a “heavy” (long) polypeptide chain and a “light” (short) polypeptide chain.

•           These chains have constant regions, located at the trunk of the Y, where the sequence of amino acids is fixed.

•           Antibodies are classified by the structure of its constant region.

•           The variable regions form an antigen-binding site; their shape is specific to a particular antigen.

•           The antigen combines with the antibody at the antigen-binding site in a lock-and-key manner.

•           Antibodies may consist of single Y-shaped molecules, called **monomers**, or may be paired together in a **dimer.** 

•           Some are **pentamers**—clusters of five Y-shaped molecules linked together.

**Classes of Antibodies**

There are five classes of circulating antibodies: **IgG, IgM, IgA, IgD, IgE.**

•           **IgG**—the major type in blood; smaller numbers are found in lymph and interstitial fluid.

•           Bind to pathogens, toxins.

•           Crosses the placenta from mother to fetus; confers temporary immune protection.

•           **IgM**—pentamers; the first antibodies produced by a newborn.

•           The first to appear in blood after an infection begins and the first to disappear when the infection is over.

•           Activate the complement system.

•           **IgA**—monomers or dimers containing two Y-shaped structures.

•           Main type of antibody found in body secretions: saliva, tears, mucus, and breast milk.

•           Bind to pathogens so they can’t reach the bloodstream.

•           **IgD**—antigen receptors on immature B cells.

•           **IgE**—prevent parasitic worm infections.

•           Can also cause allergic responses.

•           Can be brought about naturally through an infection or artificially through medical intervention.

•           There are 2 types of acquired immunity: **active** and **passive.**

**Active immunity.**

•           The individual’s body makes antibodies against a particular antigen.

•           Can happen through natural infection or through **immunization** involving **vaccines.**

•           Contain antigens from the pathogen or the pathogens themselves (treated so they can no longer cause disease).

•           The first exposure to an antigen produces a **primary response**; second exposure a **secondary response.**

•           Depends on **memory lymphocytes** and sometimes booster shots.

**Passive immunity.**

•           An individual is given antibodies to combat a disease; since not produced by the individual’s plasma cells, passive immunity is temporary.

•           That is, newborn infants are passively immune to some diseases because IgG antibodies have crossed the placenta from the mother’s blood.

•           Breast-feeding prolongs this natural passive immunity because IgG and IgA antibodies are present in mother’s milk.

•           That is, can also receive antibodies by injection.

**Hypersensitivity reactions**—when the immune system overreacts and causes harm to the body.

•           That is, allergies, receiving an incompatible blood type, tissue rejection, or autoimmune disease.

**Allergies**

•           Hypersensitivity to **allergens** such as pollen, food, or animal hair, which would normally be harmless.

•           **Immediate allergic response**—occurs within seconds of contact with the antigen.

•           Caused by IgE antibodies attached to receptors on mast cells and basophils, which release histamine when they bind allergens.

•           When pollen is an allergen, histamine stimulates the mucous membranes of the nose and eyes to release fluid (runny nose and watery eyes—hay fever).

•           With asthma, airways constrict, resulting in wheezing.

•           Food allergies—nausea, vomiting, diarrhea.

•           **Anaphylactic shock—**an immediate allergic response.

•           Occurs when the allergen enters the bloodstream.

•           Characterized by a sudden and life-threatening drop in blood pressure due to increased permeability of the capillaries by histamine.

•           Taking epinephrine can counteract this reaction.

•           **Delayed allergic responses—**initiated by memory T cells at the location of contact with the allergen.

•           That is, poison ivy.

**Tissue Rejection**

Rejection of transplanted tissue results because the recipient’s immune system recognizes that the transplanted tissue is not “self”.

•           Cytotoxic T cells attack the transplanted tissue.

•           Can be controlled with **immunosuppressive drugs** and by transplanting organs that have the same MHC proteins in the donor and recipient.

•           Some immunosuppressive drugs act by inhibiting the production of cytokines.

•           **Xenotransplantation**—transplanting organs from an animal.

•           Some organs can be grown in a lab.

Severe Combined Immunodeficiency Disease

**Severe combined immunodeficiency disease.**

•           Both antibody- and cell-mediated immunity are inadequate or lacking.

•           A minor infection can be fatal.

•           Treated with bone marrow transplant or gene therapy.

**Acquired Immune Deficiency**

Acquired immune deficiencies can be caused by infections, chemical exposure, or radiation.

•           **Acquired immunodeficiency syndrome (AIDS)** results from infection with the **human** **immunodeficiency virus (HIV).**

•           AIDS patients are more susceptible to infections and have a higher risk of cancer.

**Autoimmune disease.**

•           Cytotoxic T cells or antibodies attack the body’s own cells as if they were foreign.

•           Involves both genetic and environmental factors.

•           Sometimes follows an infection.

•           That is, **rheumatic fever**—antibodies induced by a streptococcal bacterial infection of the throat also react with heart muscle.

•           Damages the heart muscle and valves.

•           That is, **rheumatoid arthritis**—antibodies against joints.

**Systemic lupus erythematosus (SLE).**

•           Usually just called ‘lupus’.

•           Various symptoms: facial rash, fever, joint pain.

•           Damage to the central nervous system, heart, and kidneys can be fatal.

•           Produce high levels of anti-DNA antibodies.

•           All human cells (except red blood cells) contain DNA, so the symptoms of lupus can be exhibited throughout the body.

**Multiple Sclerosis.**

•           T cells attack the myelin sheath covering nerve fibers, causing central nervous system dysfunction, double vision, and muscular weakness.

•           MS may not be an autoimmune disease, because a specific antigen has not been identified.

•           Treatments for all of these diseases are drugs designed to decrease the immune response.

 

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