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Chapter 43
The Immune System
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
• 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
3 µm
• 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
INNATE IMMUNITY
Rapid responses to a
broad range of microbes
External defenses
Skin
Mucous membranes
Secretions
Invading
microbes
(pathogens)
ACQUIRED IMMUNITY
Slower responses to
specific microbes
Internal defenses
Phagocytic cells
Antimicrobial proteins
Inflammatory response
Natural killer cells
Humoral response
(antibodies)
Cell-mediated response
(cytotoxic
lymphocytes)
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
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
Plasma
Lymph
Serum55%
Constituent
Major functions
Water
Solvent for
carrying other
substances
Ions (blood electrolytes)
Sodium
Potassium
Calcium
Magnesium
Chloride
Bicarbonate
Plasma proteins
Albumin
Fibrinogen
Immunoglobulins
(antibodies)
Osmotic balance,
pH buffering, and
regulation of
membrane
permeability
Osmotic balance,
pH buffering
Clotting
Defense
Substances transported by blood
Nutrients (such as glucose, fatty acids,
vitamins)
Waste products of metabolism
Respiratory gases (O2 and CO2)
Hormones
Cellular elements 45%
Cell type
Number
Functions
per µL (mm3) of blood
Erythrocytes
(red blood cells)
Hematocrit
Separated
blood
elements
5–6 million
Leukocytes
5,000–10,000
(white blood cells)
Transport oxygen
and help transport
carbon dioxide
Defense and
immunity
Lymphocyte
Basophil
Eosinophil
Neutrophil
Platelets
Monocyte
250,000–
400,000
Blood clotting
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
Erythrocytes
• Red blood cells, or erythrocytes, are by far the most
numerous blood cells
• They transport oxygen throughout the body
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
Platelets
• Platelets function in blood clotting
platelets
fibrin
red blood cell
white blood cell
Stem Cells and the Replacement of Cellular Elements
• The cellular elements of blood wear out and are
replaced constantly throughout a person’s life
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
Platelets
Monocytes
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
LE 43-10
Bone marrow
Lymphoid
stem cell
B cell
Thymus
T cell
Blood, lymph, and lymphoid tissues
(lymph nodes, spleen, and others)
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
blood vessel
clot
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
Platelet plug
Fibrin clot
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
Red blood cell