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

Phagocyte engulfing 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

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

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

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

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 white blood cell red 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) 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

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

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)

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 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