The Immune System The Body’s Ultimate Defense

Our Lines of Defense

 First Line—Nonspecific—Skin, mucous membranes, secretions of skin and mucous membranes  Second Line—Nonspecific—Phagocytic WBCs, Antimicrobial proteins, the inflammatory response  Third Line—Specific—Lymphocytes and antibodies

First Line of Defense  Skin acts as a barrier to invaders—unless it is cut  Mucous in openings traps bacteria/viruses  Secretions such as sweat change pH to prevent microbial growth  Salvia, tears, other mucous “wash” away invaders  Secretions contain antimicrobial proteins such as lysozyme

If invaders pass the first line. . .

 Phagocytosis—Ingestion of invading organisms by certain white blood cells  Neutrophils (60 – 70% of all WBC’s)— respond to chemicals released by cells damaged by invaders  Neutrophils self-destruct doing their job & only live a few days

If invaders pass the first line. . .

 Monocytes (5% of leukocytes orWBC’s)— circulate in blood and develop into macrophages (“big eaters”)  Macrophages are the biggest WBC’s and are long lived.

 Macrophages use pseudopodia to trap invaders and then digest them  Especially abundant in the lymph system and spleen

Figure 43.4

Figure 43.3

If invaders pass the first line. . .

 Eosinophils (1.5% of all WBC’s)—protect against larger parasitic invaders such as protists and worms  Natural Killer Cells (NK cells)—destroy virus-infected body cells and abnormal body cells that could become cancerous. They cause an abnormal cell to lyse.

If invaders pass the first line. . .

  The inflammatory response is evoked Arterioles dilate, venules constrict; increasing blood supply and leaking fluid into the “infected” tissue    Chemical signals initiates inflammatory response One is histamine released by cells in response to injury Histamine is produced by WBC’s known as basophils and by mast cells in connective tissue  Chemokines secreted by vessel tissue attract phagocytes to the area.

The Inflammatory Response

Why are infected areas

 Red?

 Warm?

 Swollen?

If the injury is severe  Body issues a systemic nonspecific response  Increasing the number of leukocytes  Raising body temperature (fever) which inhibits the growth of microbes, facilitates phagocytosis, and speeds up body reactions to further healing

If invaders pass the first line. . .

   Antimicrobial proteins are released They attack microbes or impede their reproduction Complement system—serum proteins that lyse microbes & attract cells to sites of infection  Interferon—proteins secreted by virus-infected cells  Interferon induce neighboring cells to produce chemicals that inhibit viral reproduction limiting the cell to cell spread of viruses—not virus specific

Specific Immunity The Third Line of Defense

Specific Defense—The Work of Lymphocytes

 Two types—B cells & T cells  Circulate through blood and lymph  Concentrated in the spleen, lymph nodes, & lymph tissue  Recognize & respond to SPECIFIC invaders  ANTIGEN—foreign molecule that elicits a response from lymphocytes  ANTIBODIES—Proteins that interact with specific antigens

How do cells recognize antigens?

 Antigen receptors are located on cell’s plasma membranes  A single lymphocyte has about 100,000 receptors for antigen  The structure of a lymphocyte’s receptor is determined by genetic events early in its development  Immune system has millions of different antigenic molecules

Clonal Selection of Lymphocytes  An antigen binds to a specific receptor on a lymphocyte  This activates the lymphocytes specific to that antigen  Each activated cell divides giving rise to clones of thousands of cells specific for that antigen  Some B cells become memory cells that are long lived and ready to recognize that antigen the next time  This confers immunological memory

Clonal Selection of B cells

Immunological Memory— Primary vs. Secondary response

B cells vs. T cells  Both originate in the bone marrow from a pluripotent stem cells  Some migrate to the thymus for maturation and become ‘T’ cells  Some remain in the bone marrow for maturation and become ‘B’ cells  B or T cells with antigen receptors specific for molecules already present in the body are destroyed (apoptosis) or made nonfunctional  This allows our immune system to differentiate between “self” and “non-self”

Insert Figure 43.8

The Types of Lymphocytes

 B cells – Plasma Cells—Create and release antibodies – Memory Cells—Circulate in the bloodstream bearing antigen receptors specific for the same antigen  T cells – Helper T cells—Stimulates other lymphocytes by releasing chemical signals (cytokines) – Cytotoxic T cells—Directly kill cancer cells & cells infected by viruses or other pathogens

Major Histocompatibility Complex (MHC)  Glycoproteins inside cells that mark body cells as “self”  Class I MHC—found in almost all nucleated cells  Class II MHC—found in macrophages, B cells, activated T cells, and thymus cells  Cells were first discovered as they were the cause of rejection of skin grafts

MHC results in antigen presentation  Fragment of foreign protein (antigen) inside the cell associates with MHC molecule  The MHC molecules transports the antigen to the cell surface  The MHC/antigen complex is recognized by a T cell, alerting it to the infection  Class I MHC attracts cytotoxic T cells  Class I MHC attracts Helper T cells

Two Types of Immune Response  Humoral Immunity—B cell activation resulting from production of antibodies; usually protects against free bacteria, toxins, & viruses in body fluids  Cell-mediated Immunity—Immunity depending on the action of T cells; usually protects against body cells infected with viruses & bacteria, as well as fungi, protozoa & parasitic worms, transplanted tissue & cancer cells

The Functions of T cells

The functioning of cytotoxic T cells

How T cells “help” B cells

Summary of the Immune Response

Antibody = Immunonglobin

Antibody = Immunonglobin  IgM—First antibodies to appear. Presence indicates a current infection. Cannot cross placenta.

 IgG—Most abundant circulating antibody. Crosses blood vessels and the placenta giving fetus passive immunity. Protects again bacteria, viruses, and toxins; triggers complement system.

Antibody = Immunonglobin  IgA—Produced by cells in the mucous membranes. Prevents the attachment of viruses & bacteria to epithelial surfaces. Present in mother’s first milk.

 IgD—Mostly found on surface of B cells. Probably function as antigen receptors. Cannot cross placenta.

 IgE—Cause cells to release histamine and other chemicals causing allergic reactions.

How does the antigen-antibody complex give immunity?

 Neutralization—antibody binds to & blocks the activity of the antigen  The complex enhance macrophage attachment to and phagocytosis of microbes.

 Because each antibody has two antigen attachment sites, it can link microbe cells together (agglutination) which can be eaten by macrophages.

 Complex signals the release of complement

How does immunity develop?

 Active Immunity—you have infection—you develop immunity.

 Immunization/Vaccination—The intake of inactivated bacterial toxins, killed microbes, parts of microbes and viable but weakened microbes. They create memory B cells.

 Passive Immunity—When IgG antibodies of a pregnant woman passes to her child. Does not last for a long period of time.

ABO Blood Groups  Blood Types A, B, and AB have RBC that have antigens on their surface that their owners recognize as “self”. The immune system does not attack these cells.

 If transfusions are given that introduce foreign antigens, the immune system attacks those RBC.

A mother, a fetus, and Rh factor  The immune response to ABO blood groups does not produce immunological memory SO, a mother can have one blood type and her baby another blood type without her immune system attacking the fetus.

 However, if the mother is Rh negative (lacking the Rh antigen) and she bears an Rh postive baby there is a problem.

Immunity, Tissue Grafts, and Organ Transplanation  Our MHC fingerprint is so unique we cannot accept foreign cells without evoking an immune response.

 Close matches are sought.

 The immune system is suppressed with drugs.

Abnormal Immune Function  Allergies—Hypersensitive responses to certain environmental antigens (allergens). Mast cells produce large amounts of histamine which dilates & increases blood vessels. This causes sneezing, runny nose, tearing eyes, and breathing difficulty.

 Anaphylactic shock—Life-threatening reaction to injected or ingested allergens. Causes immediate drop in blood pressure; bee venom, penicillin, peanuts, fish and other foods can cause it.

 Epinephrine—counteracts this response. Allergy sufferers often carry a “pin” with them!

Abnormal Immune Function  Autoimmune Disease—When the immune system loses tolerance for self and turns against “self” molecules. Lupus, rheumatoid arthritis, diabetes, multiple sclerosis  Immunodeficiency Diseases—Severe combined immunodeficiency (SCID)