26.1 Overview of the Immune System

• The immune system fights infections and cancer to keep us healthy.

• The immune system contains lymphatic organs such as the red bone marrow, thymus, lymph nodes, and spleen.

• Lymphoid tissue can also be found in the tonsils and appendix.

Lymphatic Organs

• Each of the lymphatic organs has a particular function in immunity, and each is rich in lymphocytes, one of the types of white blood cells.

Red Bone Marrow

• Most bones in a child have red bone marrow

• In an adult, red bone marrow is still present in certain bones:

  • Skull

  • Sternum (breastbone)

  • Ribs

  • Clavicle

  • Pelvic bones

  • Vertebral column

  • Ends of the humerus and femur nearest their attachment to the body

• Red bone marrow produces all types of blood cells.

• Lymphocytes differentiate into B lymphocytes (B cells) or T lymphocytes (T cells).

• B lymphocytes are produced and mature in the bone marrow.

• T lymphocytes mature in the thymus.

• B lymphocytes produce antibodies.

• T lymphocytes kill antigen-bearing cells outright.

Thymus

• The thymus is soft and bilobed.

• The size of the thymus varies and is larger in children.

• The thymus shrinks as we get older.

• The thymus plays a role in the maturing of T lymphocytes.

• Immature T lymphocytes migrate from the bone marrow to the thymus.

• T lymphocytes mature in the thymus.

• Only 5% of T lymphocytes leave the thymus.

• T lymphocytes that react with our own body die.

• T lymphocytes that can attack foreign cells leave the thymus and enter lymphatic vessels and organs.

Lymph Nodes

• Lymph nodes are small, ovoid structures along lymphatic vessels.

• Lymph nodes filter lymph and keep it free of pathogens and antigens.

• Lymph is filtered as it flows through a lymph node because the node’s many sinuses (open spaces) are lined by macrophages.

• Macrophages are large, phagocytic cells that engulf and then devour as many as a hundred pathogens and still survive.

• Lymph nodes contain many lymphocytes and are instrumental in fighting infections and cancer.

• Some lymph nodes are located near the surface of the body and are named for their location.

• For example, inguinal nodes are in the groin, and axillary nodes are in the armpits.

• Physicians often feel for the presence of swollen, tender lymph nodes in the neck as evidence that the body is fighting an infection.

• This method is a non-invasive, preliminary way to help them make a diagnosis.

Spleen

• The spleen is located in the upper left abdominal cavity and is about the size of a fist.

• The spleen filters the blood and contains red pulp and white pulp tissue.

• The red pulp sinuses filter the blood of pathogens and debris, including worn-out red blood cells.

• The white pulp contains lymphocytes that fight infections and cancer.

• The spleen's outer capsule is thin and can burst due to infection or severe blow.

• Other organs can fulfill the spleen's functions, but individuals without a spleen are more susceptible to infections.

• People without a spleen may need to receive certain vaccinations and antibiotic therapy indefinitely.

Other Locations of Lymphoid Tissue

• Tonsils and the appendix are lymphatic tissue structures.

• Tonsils are located in the pharynx.

• The appendix is attached to a portion of the large intestine.

• Both tonsils and appendix belong to the immune system.

Cells of the Immune System

• The immune system consists of lymphatic organs, lymphatic tissues, and various cells.

• The cells in the immune system help distinguish between self and nonself cells.

• Pathogens are identified by the presence of antigens, which are usually proteins or carbohydrates.

• The immune system provides immunity by repelling foreign substances, pathogens, and cancer cells.

• There are two levels of immunity: nonspecific immunity and specific (adaptive) immunity.

• Nonspecific immunity repels pathogens indiscriminately, while specific immunity requires a certain antigen to be present.

26.2 Nonspecific Defenses and Innate Immunity

• The body has innate immunity.

• Innate immunity is composed of various types of nonspecific defenses.

• Nonspecific defenses are the first line of defense against most types of infections.

• Nonspecific defenses include:

  • Barriers to entry.

  • Inflammatory response.

  • Complement system.

  • Natural killer cells.

Barriers to Entry

• Skin and mucous membranes in respiratory, digestive, reproductive, and urinary tracts act as mechanical barriers to pathogens.

• Oil gland secretions contain chemicals that weaken or kill certain bacteria on the skin.

• The upper respiratory tract has ciliated cells that sweep mucus and trap particles up into the throat.

• The stomach's acidic pH inhibits the growth of or kills many types of bacteria.

• Normal bacteria in the large intestine and other areas prevent pathogens from taking up residence.

The Inflammatory Response

• The inflammatory response is important in defending against pathogens.

• Neutrophils and macrophages are used to surround and kill pathogens.

• Protective proteins are also involved.

• Four hallmark symptoms of inflammation are redness, heat, swelling, and pain.

• Capillary changes in the damaged area cause the four signs of inflammation.

• Chemical mediators, such as histamine, cause capillaries to dilate and become more permeable.

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

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

• Clot formation in the injured area prevents blood loss.

• Excess fluid in the area causes pain associated with swelling.

• Neutrophils are the first to arrive and actively phagocytize debris, dead cells, and bacteria.

• If neutrophils die off in great quantity, they become pus.

• If neutrophils are overwhelmed, they call for reinforcements by secreting cytokines.

• Cytokines attract more white blood cells to the area, including monocytes.

• Monocytes become macrophages, even more, powerful phagocytes than neutrophils.

• Inflammation is the body's natural response to irritation or injury.

• Chronic inflammation can last for weeks, months, or even years.

• Anti-inflammatory medications can minimize the effects of various chemical mediators.

The Complement System

• The complement system is composed of blood plasma proteins designated by the letter C and a number.

• Complement proteins complement certain immune responses, hence their name.

• They amplify the inflammatory response by binding to mast cells and triggering histamine release.

• They attract phagocytes to the site of infection.

• Some complement proteins bind to the surface of pathogens already coated with antibodies, ensuring their phagocytosis by neutrophils or macrophages.

• Certain complement proteins join to form a membrane attack complex, which creates holes in the surfaces of microbes.

• The membrane attack complex causes fluids and salts to enter the pathogen, leading to its bursting.

Natural Killer Cells

• Natural killer (NK) cells are large, granular lymphocytes that kill virus-infected cells and tumor (cancer) cells by cell-to-cell contact.

• NK cells attack and kill cells that have lost their ability to produce self-proteins.

• Cells of the body have self-proteins on their surface that bind to receptors on NK cells.

• When NK cells can't find self-proteins to bind to, they kill the cell using the same method as T lymphocytes.

• NK cells are not specific, and their numbers do not increase when exposed to a particular antigen.

• NK cells have no means of "remembering" an antigen from previous contact with it.

26.3 Specific Defenses and Adaptive Immunity

• Nonspecific defenses are the first line of defense against infections.

• If nonspecific defenses fail, specific defenses or adaptive immunity come into play.

• Specific defenses respond to antigens, which may be components of a pathogen or cancer cell.

• Antigens act as markers that a pathogen may be present in the body.

• The immune system detects the marker and begins to actively look for cells that possess the antigen.

• Lymphocytes are capable of recognizing antigens because their plasma membranes have receptor proteins.

• B lymphocytes mature in the bone marrow and give rise to plasma cells, which produce antibodies.

• T lymphocytes mature in the thymus and do not produce antibodies.

• Some T lymphocytes regulate the immune response and other T lymphocytes directly attack cells that bear antigens.

• Immunity usually lasts for some time and is primarily the result of the action of B and T lymphocytes.

• The immune system is able to distinguish self from nonself to aid, rather than disrupt, homeostasis.

B Cells and the Antibody Response

• Each B cell can only bind to a specific antigen that fits the binding site of its receptor.

• The receptor is called a B-cell receptor (BCR).

• B cells are activated when an antigen binds to their BCR.

• Activated B cells divide and produce many plasma cells and memory B cells.

• This mechanism is called the clonal selection model.

• B cells are stimulated by cytokines to divide and produce plasma cells.

• Plasma cells have extensive rough endoplasmic reticulum for mass production and secretion of antibodies.

• Antibodies produced by plasma cells are identical to the BCR of the activated B cell.

• Memory B cells are responsible for long-term immunity.

• Memory B cells quickly divide and give rise to more plasma cells capable of producing the correct antibodies if the same antigen enters the system again.

• Once the threat of an infection has passed, the development of new plasma cells ceases, and those present undergo apoptosis.

The Function of Antibodies:

• Antibodies are immunoglobulin proteins.

• They can combine with a specific antigen.

• The antigen-antibody reaction can produce antigen-antibody complexes.

• These complexes mark the antigens for destruction.

• The complexes can be engulfed by neutrophils or macrophages.

• The complexes can activate the complement system.

• The complement system makes the pathogens more susceptible to phagocytosis.

ABO Blood Type

• Antibodies can be understood by examining human blood types.

• Blood types are categorized as A, B, AB, or O based on antigens in red blood cells.

• Type O blood has both anti-A and anti-B antibodies in the plasma.

• Giving type A blood to a person with type O blood will cause agglutination.

• Type O blood can be given to a person with any blood type.

• Other red blood cell antigens are used in typing blood besides A and B.

• The donor's blood should be put on a slide with the recipient's blood to determine compatibility.

T Cells and the Cellular Response

• T cells have unique T-cell receptors (TCRs) when they leave the thymus, just like B cells.

• T cells require an antigen-presenting cell (APC) to recognize an antigen.

• Macrophages become APCs by ingesting and destroying a pathogen.

• The APC travels to a lymph node or spleen where T cells congregate.

• An antigen from a virus is combined with a major histocompatibility complex (MHC) protein after being digested in a lysosome.

• The MHC + antigen complex is presented to a T cell.

• The two main types of T cells are helper T cells (TH cells) and cytotoxic T cells (TC cells).

• TH cells recognize antigens presented by APCs with MHC class II molecules on their surface.

• TC cells recognize antigens presented by APCs with MHC class I molecules on their surface.

• Apoptosis contributes to homeostasis by regulating the number of cells present in the immune system.

• T-cell cancers can result when apoptosis does not occur as it should.

• T cells that have the potential to destroy the body's own cells undergo apoptosis in the thymus gland.

Functions of Cytotoxic T Cells and Helper T Cells

• Cytotoxic T cells specialize in cell-to-cell combat.

• They have storage vacuoles containing proteins called perforins or enzymes called granzymes.

• When a cytotoxic T cell binds to a virus-infected or cancer cell presenting the antigen it has learned to recognize, it releases perforin molecules, which perforate the target cell’s plasma membrane, forming a pore.

• The cytotoxic T cell then delivers a supply of granzymes into the pore, and these cause the cell to undergo apoptosis.

• Once cytotoxic T cells have released their perforins and granzymes, they move on to the next target cell.

• Cytotoxic T cells are responsible for a cellular response to virus-infected and cancer cells.

• Helper T cells specialize in regulating immunity by secreting cytokines that stimulate B cells and cytotoxic T cells.

• Cloned T cells include memory T cells that live for many years and can jumpstart an immune response to an antigen that was dealt with before.

• HIV infects helper T cells and other cells of the immune system, inactivating the immune response and making HIV-infected individuals susceptible to opportunistic infections.

• Infected macrophages serve as reservoirs for the HIV virus.

Tissue Rejection

• Certain organs (skin, heart, kidneys) can be transplanted from one person to another if the body does not reject them.

• Rejection occurs due to cytotoxic T cells and antibodies destroying foreign tissues in the body.

• The immune system distinguishes between self and nonself when rejection occurs.

• Organ rejection can be controlled by selecting the organ carefully and administering immunosuppressive drugs.

• The transplanted organs should have the same type of MHC proteins as the recipient to avoid being antigenic to the recipient's T cells.

• Immunosuppressive drugs inhibit T cell response to cytokines, weakening all types of immune responses.

26.4 Immunizations

• After an infection, a person may become immune to it.

• Childhood diseases like measles and mumps can provide lasting immunity.

• Sexually transmitted diseases usually do not provide lasting immunity.

• Vaccines are substances that stimulate the immune system without causing illness.

• Vaccines can be made from pathogens or genetically engineered bacteria.

• Immunization promotes active immunity, which is long-lasting.

• Active immunity is dependent on memory B and T cells.

• Passive immunity is temporary and can be obtained through prepared antibodies.

• Newborns receive passive immunity from their mothers, but it soon disappears.

• Gamma globulin injections can provide passive immunity but may cause serum sickness.

26.5 Disorders of the Immune System

• The immune system protects us from disease by distinguishing self from nonself.

• Sometimes, the immune system responds in a way that harms the body.

• Allergies and autoimmune diseases are examples of harmful immune responses.

Allergies

• Allergies are hypersensitivities to substances in the environment.

• Allergens are substances that cause an immune reaction.

• Immediate allergic responses are caused by receptors attached to the plasma membranes of mast cells in the tissues.

• Mast cells release histamine and other substances that bring about the symptoms.

• Anaphylactic shock is a severe reaction characterized by a sudden and life-threatening drop in blood pressure.

• Allergy shots may prevent the onset of an allergic response.

• Delayed allergic responses are probably initiated by memory T cells at the site of allergen contact in the body.

• A classic example of a delayed allergic response is the skin test for tuberculosis (TB).

• Contact dermatitis is also an example of a delayed allergic response.

Autoimmune Diseases

• Autoimmune diseases occur when cytotoxic T cells or antibodies attack the body's own cells as if they were antigens.

• The cause of autoimmune diseases is unknown, but they can sometimes occur after an individual has recovered from an infection.

• Myasthenia gravis is an autoimmune disease that affects neuromuscular junctions, causing muscular weakness.

• Multiple sclerosis (MS) is an autoimmune disease that causes the breakdown of the myelin sheath of nerve fibers, resulting in various neuromuscular disorders.

• Systemic lupus erythematosus causes various symptoms prior to death due to kidney damage.

• Rheumatoid arthritis affects the joints.

• Rheumatic fever and type 1 diabetes are suggested to be autoimmune illnesses.

• There are currently no cures for autoimmune diseases, but they can be controlled with drugs.

AIDS

• AIDS is caused by HIV, which destroys helper T cells in the immune system.

• Without helper T cells, the immune system can no longer fight off viruses, fungi, and bacteria.

• Symptoms of AIDS include weight loss, chronic fever, cough, diarrhea, swollen glands, and shortness of breath.

• HIV is transmitted through sexual contact, needle sharing, and from mother to child during birth or breastfeeding.

• Advances in treatment have prolonged life, but new strains of the virus have emerged that are resistant to drugs.

• AIDS vaccine trials are underway, but the process can take many years.

• The human body's ability to suppress the infection keeps scientists hopeful that there is a way to help the body overcome HIV infection.

• Suggestions for preventing HIV infection include abstaining from sexual intercourse, using a condom, and avoiding risky behavior such as intravenous drug use.