The Immune System

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Chapter 43

• The Immune System

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Overview Reconnaissance, Recognition, and
Response

• An animal must defend itself from the many
  dangerous pathogens it may encounter
• Pathogens are infectious agents that cause
  disease
• Two major kinds of defense have evolved
• immunity defenses to avoid infection,
• disease, or other unwanted biological
• invasion
• innate immunity
• acquired immunity

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• Innate immunity is present before any exposure to
  pathogens and is effective from the time of birth
• It involves nonspecific responses to pathogens
• Innate immunity consists of external barriers
  plus internal cellular and chemical defenses
• Key internal defenses are macrophages and other
  phagocytic cells

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3 µm
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• Acquired immunity, or adaptive immunity, develops
  after exposure to pathogens such as microbes,
  toxins, or other foreign substances
• It involves a very specific response to pathogens
• Recognition is by white blood cells called
  lymphocytes
• Some lymphocytes produce antibodies others
  destroy infected cells, cancer cells, or foreign
  tissue

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ACQUIRED IMMUNITY Slower responses to specific
microbes
INNATE IMMUNITY Rapid responses to a broad range
of microbes
External defenses
Internal defenses
Skin
Phagocytic cells
Humoral response (antibodies)
Mucous membranes
Antimicrobial proteins
Secretions
Inflammatory response
Invading microbes (pathogens)
Cell-mediated response (cytotoxic lymphocytes)
Natural killer cells
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Blood Composition and Function

• Blood consists of several kinds of cells
  suspended in a liquid matrix called plasma
• The cellular elements occupy about 45 of the
  volume of blood

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Plasma

• Blood plasma is about 90 water
• Among its solutes are inorganic salts in the form
  of dissolved ions, sometimes called electrolytes
• Another important class of solutes is the plasma
  proteins, which influence blood pH, osmotic
  pressure, and viscosity
• Various plasma proteins function in lipid
  transport, immunity, and blood clotting

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Plasma 55
Serum
Lymph
Cellular elements 45
Constituent
Major functions
Cell type
Number
Functions
Water
Solvent for carrying other substances
per µL (mm3) of blood
Erythrocytes (red blood cells)
Ions (blood electrolytes)
56 million
Transport oxygen and help transport carbon dioxide
Sodium Potassium Calcium Magnesium Chloride Bicarb
onate
Osmotic balance, pH buffering, and regulation
of membrane permeability
Hematocrit Separated blood elements
Leukocytes (white blood cells)
Defense and immunity
5,00010,000
Plasma proteins
Osmotic balance, pH buffering
Albumin
Lymphocyte
Basophil
Fibrinogen
Clotting
Defense
Immunoglobulins (antibodies)
Eosinophil
Substances transported by blood
Monocyte
Neutrophil
Nutrients (such as glucose, fatty acids,
vitamins) Waste products of metabolism Respiratory
gases (O2 and CO2) Hormones
Platelets
250,000 400,000
Blood clotting
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Cellular Elements

• Suspended in blood plasma are two types of cells
• Red blood cells (erythrocytes) transport oxygen
• White blood cells (leukocytes) function in
  defense
• Platelets, a third cellular element, are
  fragments of cells that are involved in clotting

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Erythrocytes

• Red blood cells, or erythrocytes, are by far the
  most numerous blood cells
• They transport oxygen throughout the body

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Leukocytes

• There are five major types of white blood cells,
  or leukocytes monocytes, neutrophils, basophils,
  eosinophils, and lymphocytes
• They function in defense by phagocytizing
  bacteria and debris or by producing antibodies

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Platelets

• Platelets function in blood clotting

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Stem Cells and the Replacement of Cellular
Elements

• The cellular elements of blood wear out and are
  replaced constantly throughout a persons life

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Pluripotent stem cells (in bone marrow)
Myeloid stem cells
Lymphoid stem cells
Erythrocytes, leukocytes, and platelets all
develop from a common source, pluripotent stem
cells in the red marrow of bones
Basophils
B cells
T cells
Lymphocytes
Eosinophils
Neutrophils
Erythrocytes
Monocytes
Platelets
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Lymphocyte Development

• Lymphocytes arise from stem cells in bone marrow
• Newly formed lymphocytes are alike but later
  develop into B cells or T cells, depending on
  where they mature

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LE 43-10
Bone marrow
Thymus
Lymphoid stem cell
B cell
T cell
Blood, lymph, and lymphoid tissues (lymph nodes,
spleen, and others)
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Blood Clotting

• When the endothelium of a blood vessel is
  damaged, the clotting mechanism begins
• A cascade of complex reactions converts
  fibrinogen to fibrin, forming a clot

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LE 42-17
Endothelium of vessel is damaged, exposing
connective tissue platelets adhere
Platelets form a plug
Seal is reinforced by a clot of fibrin
Collagen fibers
Fibrin clot
Red blood cell
Platelet plug
Platelet releases chemicals that make nearby
platelets sticky
Clotting factors from
Platelets Damaged cells Plasma (factors include
calcium, vitamin K)
Prothrombin
Thrombin
Fibrinogen
Fibrin
5 µm