The Body’s Defenses Ch. 43

While some organisms can live symbiotically within animals, there are MANY that cause harm Humans have a three level defense system:

First Line of Defense Nonspecific defense Skin: Physical barrier covered with acidic secretions (pH 3-5) Mucus: Viscous fluid excreted by cells of mucous membranes traps microbes and other particles Cilia: Line the lungs and “sweep” invaders out Gastric Juice: Kills most microbes in stomach Antimicrobial Proteins: Protective proteins Ex: Lysozyme digests cell walls of many bacteria Symbiotic Bacteria: out compete other organisms that could cause harm (ex: some found in digestive tract)

Second Line of Defense Nonspecific defense Phagocytes WBCs (leukocytes) Use phagocytosis – process of “ingest and destroy” Neutrophils Most common phagocyte Short life span 60-70% of WBCs Cells damaged by invading microbes release signals that attract neutrophils Neutrophils enter the infected tissue, engulfing and destroying microbes there

Monocytes Another type of phagocyte Only 5% of WBCs More effective than neutrophils Grow into bigger phagocytic cells called macrophages Macrophages: Effective, long-lived phagocytes Natural Killer Cells (NK Cells) Not phagocytic Instead they attack a cell’s membrane, causing it to lyse (burst open) Destroy virus-infected body cells and abnormal body cells

Antimicrobial Proteins “Complement” is a group of proteins that help defense reactions Help attract phagocytes to foreign cells Help destroy foreign cells by promoting lysis Interferons Secreted by cells invaded by viruses Stimulate neighboring cells to make proteins that will help them defend against the viruses

The Inflammatory Response Triggered by: Damage to tissue by a physical injury Entry of microorganisms 1) Injured tissue releases chemical signals like histamine which is secreted by basophils Basophils: WBC’s found in connective tissue 2) Histamine stimulates vasodilation in nearby capillaries, making them more permeable for clotting factors and other fluids. Clotting begins.

3) Factors released by various kinds of cells attract 3) Factors released by various kinds of cells attract phagocytic cells from the blood 4) Phagocytic cells consume pathogens and cell debris. “Complement” proteins help the phagocytic cells, stimulate basophils to release histamine, and help lyse foreign cells. The tissue heals.

Simplified View of Inflammatory Response

Third Line of Defense The Immune Response Different from the first two lines of defense in that it is SPECIFIC Antigen: Any molecule that can be identified as foreign MHC Major Histocompatibility Complex Glycoproteins on the membranes of all body cells MHCs are unique to the individual (except for iden. twins) Allows immune system to recognize difference between self and non-self cells

Lymphocytes The key cells of the immune system Generate efficient & selective immune responses Two main types: B cells & T cells B cells Also called “B lymphocytes” Originate & mature in bone marrow Respond to antigens Surface of B cell membranes have antigen receptors called antibodies

Protein that is specific to a certain antigen Antibodies Protein that is specific to a certain antigen Also referred to as immunoglobulins Five classes: IgA, IgD, IgE, IgG, IgM Each class has a different role Each is a variant of a basic y-shaped protein The “variable” region of the antibody is what makes that particular antibody unique to a specific antigen Antibodies work by binding to antigens and inactivating them After inactivation: Macrophages use phagocytosis Antibodies stimulate “complement” proteins to lyse pathogens

Plasma Cells: Type of B Cell that secretes antibodies Memory Cells: So back to B Cells. When they have antigens bind to their antibodies, they produce two kinds of daughter B Cells: Plasma Cells: Type of B Cell that secretes antibodies Memory Cells: Long-lived B Cells formed during the primary immune response. Circulate through the body, don’t release their antibodies yet. Ready to respond quickly to any future exposure to the same antigen. Activated memory cells mount the secondary immune response. Can result in immunity after a first exposure.

T Cells Lymphocytes that come from bone marrow Mature in the thymus gland Membranes have antigen receptors Not antibodies, but recognition sites that are specific for molecules displayed by non-self cells MHC markers on the membrane of cells distinguish between self & non-self Cancer cells & transplant cells are recognized as non-self Cells that have been infected by a virus show both self & non-self markers, but T Cells recognize this as non-self

When T Cells find non-self cells, they produce two kinds of cells: Cytotoxic T Cells (Killer T Cells): Bind to and lyse infected cells or cancer cells, killing them Helper T Cells: Stimulates production of B cells & cytotoxic T cells

Clonal Selection The antigen-driven cloning of lymphocytes When an antigen binds to a specific receptor it is selectively activating a small fraction of cells from the HUGE pool of lymphocytes The small number of “selected” cells will grow into thousands that are specific for killing that antigen

Clonal Selection

Humoral Immune Response Responds to antigens B Cells make plasma cells & memory cells Plasma cells release antibodies Memory cells are for future use If antigen does not stimulate production of B Cells: Macrophage takes in antigen T cells bind to macrophage Helper T cells give off interleukins that stimulate B cell production

Cell-mediated Immune Response Doesn’t involve antibodies Uses mostly T cells Responds to non-self cells T cells make cytotoxic T cells & helper T cells Helper T cells bind to macrophages They bind to macrophages that have taken in antigens, because they present as “non-self” Helper T cells make interleukins which stimulate the growth of T cells & B cells Animation: The Immune Response

Other Immunity Vaccines Passive Immunity Stimulate production of memory cells Inactivated viruses/bacteria are used as vaccines Gives the immune system a “sneak preview” of an antigen Now if an individual is exposed in the future there will be a swift response before disease can set in Passive Immunity Getting antibodies from someone who previously had a disease Newborns can have passive immunity from antibodies in mothers breast milk and antibodies that crossed placenta

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