BIOL 1040 - Chapter 24

Immune System

a system of organs, tissues, cells, and cell products that protect an organism from foreign & potentially harmful organisms, tissues or other substances. 

What is the immune compromised of

The lymphatic system (including lymph vessels & nodes; thymus, spleen, tonsils & adenoids). 

Bone marrow & a variety of white blood cells. 

Antibodies & complement proteins.

A variety of signal molecules. 

Innate Immunity 

Aka non-specific, genetic, or natural immunity. 

Involves a set of defenses that are immediately active, and independent of previous encounters with a given pathogen or substance.

 Are the same, whether or not the pathogen has been previously encountered. 

Found in both vertebrates & invertebrates

Adaptive Immunity 

Aka acquired immunity, specific 

Involves a set of defenses developed for, and activated by specific pathogens or substances. 

Immune system “remembers” (or specifically recognizes) a previously encountered pathogen. 

Found only in the Vertebrates.

What does Innate immunity (that all animals have) include

Exoskeleton – a hard impermeable chitinous “shell” that prevents the entry of foreign microbes. 

Low internal pH – that inhibits the growth of microbes. 

Lysozyme – an enzyme that degrades bacterial cell walls, causing them to lyse and die. 

Phagocytes – circulating immune cells that engulf and digest foreign substances. 

Recognition proteins – bind to microbes & trigger the release of antimicrobial peptides.

First Line of Defense

Barriers to Entry: 

Skin – dead, keratinized cells form an impermeable barrier. 

Mucous membranes – form a sticky trap for microbes; ciliated cells sweep mucus & microbes into throat. 

Second line of defense

Innate Immune Cells: 

Neutrophils – “first-responder” phagocytes circulate freely in blood; are attracted to infection sites; squeeze between cells of capillary wall, which is called extravasation. 

Macrophages – “large” phagocytes that reside in, & wander through the interstitial fluids. 

Natural Killer Cells – attack cancer cells and virus-infected cells.

Interferons

Antiviral proteins produced by virus-infected cells help limit virus cell-to-cell spread. 

“Interfere” with viral replication; & activate immune cells (e.g., natural killer cells and macrophages).

Complement System 

A group of about 30 kinds of serum proteins that can act with (i.e., “complement”) or help antibody & phagocyte defense mechanisms. 

helps stimulate the inflammatory response. 

Complement Proteins

a group of about 30 different blood plasma proteins that “complement” or enhance immune responses. 

Produced mainly by liver. 

Normally circulate in bloodstream.

Must be activated during an immune response. 

Complement proteins enhance immune responses in three ways: 

1. Enhance Inflammation Bind to mast cells & trigger more histamine release. Mast cells are bone marrow-derived cells that are found resident in tissues throughout the body, particularly in association with structures such as blood vessels and nerves, and close to surfaces that interface the external environment. 

2. Help Target Cells for Destruction Bind to cells coated with antibodies as extra signal to phagocytes. Mast cells release histamines that trigger the inflammatory response. 

3. Form Membrane Attack Complexes. Channels formed by the Membrane Attack Complexes pass through the membrane(s) and cell wall of the microbe; cells cannot control the entry & exit of water & solute which leads to cell lysis and/or death.

Inflammatory Response 

The characteristic redness, swelling, pain and heat exhibited by a tissue when it is damaged by physical or chemical agents or by infection.

 Describe the steps of the inflammatory response in order

1.Tissue injury allows entry microbes. 

2. Capillaries dilate, become leaky, allow fluids into tissue. 

3. Neutrophils & macrophages phagocytize microbes & damaged cells 

Tissue injury allows entry microbes.

Mast cells release histamines – cause capillary dilation; attracts neutrophils. 

Macrophages release cytokines – promote blood flow into area. 

Damaged cells release prosta-glandins – stimulate local pain. 

Capillaries dilate, become leaky, allow fluids into tissue.

Tissue swells, becomes red & warm. 

More neutrophils are attracted to infected area. 

Platelets & clotting factors bind to damaged tissue & initiate clotting. 

 Neutrophils & macrophages phagocytize microbes & damaged cells 

Blood clot forms & tissue heals.

General Characteristics of Adaptive Immunity 

Only found in the Vertebrates. 

A second line of defense – when innate (non-specific) immune response fails to ward off a pathogen. 

Slower than innate defenses – i.e. normally takes 5 – 7 days to mount a response). 

Highly specific – involves the recognition of foreign cells, particles & molecules (i.e. “antigens”) as “non-self” by the immune system. 

Recognition is mediated by antibodies. Can “remember” antigens or pathogens that have been encountered before. 

Involves B Cell & T Cell Lymphocytes. Often referred to as “acquired immunity”.

Vaccinations Are A Great Example of

Adaptive Immunity

Active Immunity

long-term immunity that occurs when an individual produces their own supply of antibodies, either by exposure to a foreign entity or by immunization. 

e.g. Exposure to Chickenpox or Measles. 

e.g. Vaccination for Smallpox or Tetanus. 

Passive Immunity

temporary immunity that occurs when an individual is given previously prepared antibodies to prevent a disease. 

e.g. Treatments for rabies & snakebites (anti-venom). 

Mother’s antibodies protect unborn children. 

Colostrum provides antibodies to breastfed infants.

Importance of Lymphatic system

The Lymphatic System Becomes a  Crucial Battle-ground During Infection. 

Lymphatic System

Is comprised of lymph vessels & nodes, thymus, spleen, tonsils & adenoids. 

Functions closely with the circulatory system. 

Has several important functions: 

• Digestive Function (From before) . . . Lymphatic vessels of the small intestines (lacteals) absorb fats (lipoproteins) at the intestinal villi and transport them to the bloodstream. 

• Circulatory Function – Lymphatic vessels absorb excess tissue fluid and return it to the bloodstream. 

• Immune Function – Produce, maintain, & distribute lymphocytes; macrophages destroy foreign materials; help defend the body against disease. 

Thymus

produces thymosins (hormones that promote maturation of T cells). 

Spleen

helps to “immunologically filter” the blood; it contains resident macrophages that remove old/defective RBC’s, and lymphatic tissues containing B & T cell lymphocytes:

Structure & Function of Lymph Nodes 

Nodules of lymphatic tissue inter-spersed throughout the lymphatic system. 

Lymphatic fluid is “filtered” by lymph nodes where resident macrophages engulf & destroy any foreign debris and/or invaders). 

Lymphocytes may mount an adaptive immune response. 

“Swollen sensitive glands” (i.e. lymph nodes) are a sign that the body is fighting off an infection.

Tonsils & Adenoids

comprised of “lymphoid” tissue (like lymph nodes), & contain lymphocytes, macrophages & antibodies. 

trap, destroy & serve as a defense against ingested or inhaled bacteria & viruses. 

Become swollen, irritated & infected when very active. 

The Humoral Immune Response 

B cells release antibodies that tag (mark) antigens in body fluids for destruction. 

Humoral – of or pertaining to the fluids of the body, particularly the blood fluids.

The Cell-mediated Immune Response. 

Involves T Cell Lymphocytes that 

• Attack body cells infected with bacteria or viruses. 

• Promote phagocytosis by other white blood cells. 

• Stimulate B Cell Lymphocytes to produce antibodies. 

T cells actively engage & destroy infected cells of the body.

The Development of B Cells vs. T Cells

B cells mature in marrow. 

T cells mature in thymus.

Clonal Selection

The process whereby genetically unique (naïve or virgin) lymphocytes react with (recognize) their specific antigen for the very first time. 

Describe the primary and secondary immune responses, speed, strength, and duration

Many vaccinations are given in two doses.  The second is referred to as the “booster shot”.  It is given to increase the level of protection.

Characteristics of Antigens:

They are molecules (or parts of molecules) that elicit the adaptive immune response.

They usually do not belong to the host animal.

They are typically proteins or large polysaccharides on the surfaces of viruses or foreign cells.

Epitopes

specific regions on an antigen where antibodies bind.

An antigen usually has several different epitopes.

The antigen-binding site of an antibody and an antigenic determinant have complementary shapes

Structure & Function of Antibodies 

Y-shaped protein receptor molecules (aka Immunoglobins) produced by B cell lymphocytes. 

Comprised of four polypeptide chains:  Two heavy + two light chains, Each with variable & constant regions. 

Each recognizes or binds only one specific antigen. 

All function to “neutralize” antigens and/or mark them for destruction via phagocytes 

Phagocytes recognize & bind/attach to base of “Y”.

Four Ways that Antibodies Mark Antigens for Elimination 

1. Neutralization (blocks viral binding sites; coats bacteria) 

2. Agglutination (of microbes) 

3. Precipitation (of dissolved antigens) 

4. Activation of the complement system

The Activation of Helper T Cells, Cytotoxic T Cells & B Cells 7 steps

1. Macrophage ingests & degrades foreign cells or material, and breaks it into antigenic fragments.

2. Self-proteins bind to foreign non-self molecules/antigens.

3. Self/non-self complexes are displayed at cell surface.

4. Virgin helper T cells bind to self/non-self complexes (= recruitment /activation). 

5. Signal molecules stimulate division of activated Helper T cells to form memory T cells and effector T cells.  

6. Activated Helper T cells secrete Interleukin-2 which . . . AND: Activated HT cells bind to “presented antigens” on B cells, which activates them to promote division & formation of plasma cells that release antibodies. Interluekin-1 tends to stimulate the inflammatory response, while IL-2 tends to stimulate proliferation of HT cells.

7. Activates B cells (& the humoral response), and Cytotoxic T Cells (& the cell-mediated response).

How a Cytotoxic T Cell Kills an Infected Cell (4 steps)

1. A cytotoxic T cell binds to an infected cell.

2. Perforins form holes in the infected cell’s membrane, and enzymes trigger cell death.

3. The infected Cell dies.

4. Cytotoxic T cell detaches and can destroy other cells infected by the same pathogen.

The Immune System Depends on Our Molecular Fingerprints.

All cells of the human body display the same set of “self” proteins (aka as “MHC” proteins) that serve as a “molecular fingerprint”. 

Humans produce about 14 “major” complexes.

No two humans (except identical twins) produce the same sets of MHC proteins. 

The immune system recognizes “non-self” & mounts an attack – e.g. on organ transplants.

Transplant surgeons look for donors with matching MHC proteins.

Common Immune Disorders Include

• Autoimmune Diseases – the immune system fails to distinguish between foreign antigens and “self” antigens; Cytotoxic T cells & antibodies attack the bodys own cells.

• Immunodeficiency Diseases (besides AIDS) where an immune response is defective or absent.

• Allergies – fast, hypersensitive, overreactions of the immune system to normally harmless antigens.

Rheumatoid Arthritis:

Inflammation of cartilage & bone of joints, leading to their destruction, weakening of tendons & ligaments, & joint deformations.

Type 1 (Juvenile) Diabetes

Insulin-producing cells of pancreas (islets) are destroyed; blood glucose levels can not be regulated; insulin injections are required.

Allergic Reactions Occur In Two Stages:

Stage 1 – Sensitization 

Stage 2 – Over Reaction

Stage 1 – Sensitization 

– i.e. the very first exposure to an allergen – i.e. the antigen that elicits the allergen.

Allergen enters the bloodstream.

Virgin B Cells are recruited to produce antibodies.

B Cells develop into plasma cells that release antibodies.

Antibodies attach to mast cells

Stage 2 – Over Reaction

 i.e. over-stimulation of the inflammatory response during a later exposure to the same antigen. Can be life-threatening or fatal.

Allergen enters the body a second time & binds to antibodies on mast cells. 

Mast cells release histamine, which triggers allergy symptoms, including the inflammatory response.  Inflammation of nasal membranes results in typical allergy symptoms (runny nose, tearing eyes, sneezing). 

Anaphylactic Shock

an immediate & severe allergic reaction that occurs when an allergen enters the bloodstream and affects the entire body.