1. Chapter 43
The Immune System

2. 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

3. 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

4. Phagocyte engulfing
3 µm

5. 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

6. broad range of microbes
INNATE IMMUNITY
Rapid responses to a
broad range of microbes
ACQUIRED IMMUNITY
Slower responses to
specific 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

7. 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

8. 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

9. Lymph
Serum
Plasma 55%
Constituent
Major functions
Cellular elements 45%
Water
Solvent for
carrying other
substances
Cell type
Number
Functions
per µL (mm3) of blood
Erythrocytes
(red blood cells)
Ions (blood electrolytes)
5–6 million
Transport oxygen
and help transport
carbon dioxide
Sodium
Potassium
Calcium
Magnesium
Chloride
Bicarbonate
Osmotic balance,
pH buffering, and
regulation of
membrane
permeability
Hematocrit
Separated
blood
elements
Leukocytes
(white blood cells)
Defense and
immunity
5,000–10,000
Plasma proteins
Albumin
Osmotic balance,
pH buffering
Lymphocyte
Fibrinogen
Clotting
Basophil
Immunoglobulins
(antibodies)
Defense
Eosinophil
Substances transported by blood
Neutrophil
Monocyte
Nutrients (such as glucose, fatty acids,
vitamins)
Waste products of metabolism
Respiratory gases (O2 and CO2)
Hormones
Platelets
250,000–
400,000
Blood clotting

10. 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

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

12. 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

13. Platelets Platelets function in blood clotting platelets fibrin
white blood cell
red blood cell

14. Stem Cells and the Replacement of Cellular Elements
The cellular elements of blood wear out and are replaced constantly throughout a person’s life

15. Pluripotent stem cells
(in bone marrow)
Lymphoid
stem cells
Myeloid
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

16. 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

17. Blood, lymph, and lymphoid tissues (lymph nodes, spleen, and others)
Bone marrow
Thymus
Lymphoid
stem cell
B cell
T cell
Blood, lymph, and lymphoid tissues
(lymph nodes, spleen, and others)

18. 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

19. 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
Red blood cell
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