The Immune System

Recognition and Response• Pathogens: agents that cause disease, infect a wide

range of organisms• Immune system: recognizes foreign bodies and

responds with immune cells and proteins• Innate immunity: a defense active immediately upon

infection, all animals• Vertebrates also have adaptive immunity• Adaptive (or acquired) immunity develops after

exposure to agents such as microbes, toxins, or other foreign substances

• It involves a very specific response to pathogens

Overview of the Immune System

• Nonspecific Defense MechanismsFirst line of Defense• Skin• Mucous membranes and their secretionsSecond line of Defense• Phagocytic white blood cells• Natural killer cells• Antimicrobial proteins• The inflammatory response• Specific Defense Mechanisms Third Line of Defense• Lymphocytes• Antibodies

First Line of Defense – Skin and Mucous

• Skin: A barrier preventing bacteria and viruses from entering the body.

• Mucous membranes line the digestive, respiratory, and genitourinary tracts

• The skin and mucous membranes counter pathogens with chemical defenses.

• Saliva, tears, and mucous secretions wash away many potential invaders.

• In addition, these secretions contain various antimicrobial proteins.

Second Line of Defense- Phagocytic White Blood Cells

• The body’s main non-specific defense depends on phagocytosis

• The phagocytic cells called neutrophils compose about 60-70% of all white blood cells.

• Attracted by chemical signals, the neutrophils can leave the blood and destroy microbes.

• Neutrophils self-destruct as they destroy foreign invaders, and their average life is only two days.

Second Line of Defense- Phagocytic White Blood

Cells• Macrophages, which make up

5% of white blood cells, provide an even more effective phagocytic defense.

• They start out as monocytes, and then enlarge and develop into macrophages.

• These cells pull in microbes, and then destroy the microbes by digestive enzymes within the macrophages.

Second Line of Defense- Phagocytic White Blood

Cells• 1.5% of white cells are eosinophils,

for defense against larger parasitic invaders, such as worms.

Second Line of Defense -Natural Killer Cells

• Don’t attack microorganisms directly, but destroy body cells infected with viruses

• These cells infuse target cell with chemicals called perforins, and cause it to lyse

Second Line of Defense - Antimicrobial Proteins

• Attack microorganisms directly or by inhibiting their growth.

• Complement proteins and interferons are the important types of antimicrobial proteins

• Complement proteins work together to lyse invading microbes.

• They also act as attractants to lure phagocytes.

Second Line of Defense - Antimicrobial Proteins

• Interferons are substances that help prevent the spread of viruses to other cells

• Interferons are excreted by an infected cell as an early non-specific defense, before specific defenses occur.

• Interferons do not save the infected cell, instead they spread to neighboring cells to stimulate production proteins that inhibit viral replication in those cells.

Second Line of Defense -Inflammatory Response

• When small blood vessels in the area of the injury dilate and increase blood supply to the injured area

• Chemical signals are carried by various types of white cells:• Histamines (initiates inflammatory response)• Prostaglandins (increases blood flow)• Clotting elements (clot the blood)• Pyrogens (increase temperature)

• The most important part of this response is that it attracts neutrophils and monocytes (that later develop into macrophages).

• The neutrophils self-destruct after eliminating many microorganisms and the macrophages devour the pathogens and the remains of tissue cells after the self-destructions occur.

Third Line of Defense: Immune System

• Four features: specificity, diversity, self/nonself recognition, and memory.

• Specificity: The immune system has the ability to recognize & eliminate particular microorganisms.

• A substance that provokes this response is called an antigen.

• The immune responds to an antigen by making specific proteins called antibodies.

• With this type of immune system, each response of the immune system targets a specific invader.

Humoral (antibody-mediated) immune response Cell-mediated immune response

Antigen (1st exposure)

Engulfed by

Antigen-presenting cell

Helper T cell

Memoryhelper T cells

Antigen (2nd exposure)

B cell

Plasma cells

Secretedantibodies

Defend against extracellularpathogens

Memory B cellsMemory

cytotoxic T cellsActive

cytotoxic T cells

Defend against intracellularpathogens and cancer

Cytotoxic T cell

KeyStimulatesGives rise to

Figure 43.20

Third Line of Defense: Immune System

• Diversity refers to the ability of the immune system to deal with millions of kinds of invaders

• Self/nonself recognition refers to the immune system’s ability to distinguish the body’s own molecules from foreign molecules (antigens).

• Failure of this recognition leads to autoimmune disorders, where the body destroys itself.

Third Line of Defense: Immune System

• Memory refers to how the immune system remembers antigens and knows how to react against them very quickly and effectively in later exposures. This is called acquired immunity.

•Active acquired immunity is immunity achieved by one’s own immune system, or by vaccines

•Passive acquired immunity occurs when antibodies are passed from the mother to her infant. Passive immunity is temporary and the infant’s own immune system then takes over the defense of the body.

Two Responses to Antigens • Lymphocytes are responsible for the immune

response• Initially, all are alike, but differentiate into T or B

cells, depending on their function.• Two classes of lymphocytes

– B Cells mature in the bone marrow, humoral immunity

– T Cells mature in the thymus, cell-mediated immunity• Humoral Immunity: B cells produce antibodies,

which circulate as proteins in the blood and attack antigens.

• Cell-mediated Immunity: Cytotoxic T cells attack infected cells.

Two Responses to Antigens • Lymphocytes released from the bone marrow and move to the

thymus, develop into T cells (T for thymus). • Lymphocytes that remain in the bone marrow and finish

maturating there become B cells.• T cells destroy antigens that have been tagged by antibodies or

cells that have been infected or somehow changed. T cells are also involved in helping signal other cells (like phagocytes) to do their jobs.

• B cells produce antibodies that lock onto antigens. B cells produce antibodies to fight infection

T cells attacking an antigen marked with antibodies

Review• Nonspecific Defense MechanismsFirst line of Defense• Skin• Mucous membranes and their

secretionsSecond line of Defense• Phagocytic white blood cells

- Neutrophils- Macrophages- Eosinphils

• Antimicrobial proteins- Complement - Interferons

• The inflammatory response• Natural Killer Cells

•Specific Defense Mechanisms Third Line of Defense•Lymphocytes

–B cells (secrete antibodies)–T cells (killer cells)–Helper T cells (support both humoral and cell-mediated

Humoral (antibody-mediated) immune response Cell-mediated immune response

Antigen (1st exposure)

Engulfed by

Antigen-presenting cell

Helper T cell

Memoryhelper T cells

Antigen (2nd exposure)

B cell

Plasma cells

Secretedantibodies

Defend against extracellularpathogens

Memory B cellsMemory

cytotoxic T cellsActive

cytotoxic T cells

Defend against intracellularpathogens and cancer

Cytotoxic T cell

KeyStimulatesGives rise to

Figure 43.20

Immune System Problems• “Self” cells are identified MHC I (Major Histocompatability

complex I)• Self-tolerance is developed early in development when all antigen

receptors for “self” cells are destroyed• Autoimmune Diseases: Body recognizes own cells as antigenic

(lupus, rheumatoid arthritis)• Tissue Transplants: Foreign MHC molecules are seen as antigens.

Requires immunosuppressant drugs.• Severe Combined Immunodeficiency (SCID): Severe defect in both

the T- & B-lymphocyte systems. Usually results in the onset of one or more serious infections within the first few months of life. These infections are usually serious, and may even be life threatening, they may include pneumonia, meningitis or bloodstream infections.

Immune System Problems• Allergies: Hypersensitivity to environmental antigens. Severe

cases- anaphylactic shock• Severe Combined Immunodeficiency (SCID): Severe defect in

both the T- & B-lymphocyte systems. Usually results in the onset of one or more serious infections within the first few months of life. These infections are usually serious, and may even be life threatening. Most common is “Boy in the Bubble Disease”, a sex-linked trait.

• AIDS (Acquired Immunodeficiency Syndrome): Caused by HIV, which infects T cells. Antigens of the virus mutate rapidly. Immune response is initially effective, but it later fails. AIDS is the final stage of infection-T cell count is low and cell mediated response stops functioning.

Figure 43.23