Guyton 34 -M 9.1 - Leukocytes and Phagocytosis

• The body is constantly exposed to bacteria, viruses, fungi, and parasites in areas like the skin, mouth, and respiratory system.
• Many infectious agents can cause physiological dysfunction or death if they invade deeper tissues.
• Besides normal flora, acute lethal diseases can be caused by highly infectious bacteria and viruses.
• The body has a system using blood leukocytes (WBCs) and leukocyte-derived tissue cells to combat infectious and toxic agents.
• Disease is prevented by destroying invaders via phagocytosis, and forming antibodies and sensitized lymphocytes
• This chapter discusses the pathogen destruction using phagocytosis

Leukocytes (White Blood Cells)

• Leukocytes, or white blood cells, are mobile units of the body's protective system.
• Granulocytes, monocytes and some lymphocytes are formed in bone marrow, while lymphocytes and plasma cells are formed in lymph tissue.
• WBCs are transported in the blood to different parts of the body as needed.
• WBCs are transported to areas of serious infection and inflammation to defend against infectious agents.
• Granulocytes and monocytes have the ability to seek out and destroy foreign invaders.

General Characteristics of Leukocytes

• Six types of WBCs normally present in the blood exist, which are neutrophils, eosinophils, basophils, monocytes, lymphocytes and plasma cells.
• Platelets exist as fragments of cells similar to WBCs found in bone marrow.
• Polymorphonuclear cells have a granular appearance.
• Polymorphonuclear cells are called granulocytes.
• Granulocytes and monocytes protect the body by ingesting organisms via phagocytosis or releasing antimicrobial or inflammatory substances.
• Lymphocytes and plasma cells function mainly in connection with the immune system.
• Platelets activate the blood-clotting mechanism.

Concentrations of Different White Blood Cells in Blood

• An adult human has about 7000 WBCs per microliter of blood, in comparison to 5 million red blood cells per microliter.
• Normal percentages of different WBC types:
• Neutrophils: 62.0%
• Eosinophils: 2.3%
• Basophils: 0.4%
• Monocytes: 5.3%
• Lymphocytes: 30.0%
• The number of platelets is normally between 150,000 and 450,000, averaging about 300,000 per microliter of blood.

Genesis of White Blood Cells

• Early differentiation of the multipotential hematopoietic stem cell forms into the different types of committed stem cells.
• Aside from cells committed to form RBCs, two major lineages of WBCs are formed, the myelocytic and lymphocytic lineages.
• The myelocytic lineage begins with the myeloblast, while the lymphocytic lineage begins with the lymphoblast.
• Granulocytes and monocytes are formed only in the bone marrow.
• Lymphocytes and plasma cells are produced mainly in lymphogenous tissues, such as lymph glands, spleen, thymus, tonsils, and lymphoid tissue.
• WBCs formed in the bone marrow are stored there until needed in the circulatory system, and are released when needed.
• About three times as many WBCs are stored in the marrow as circulate in the entire blood, representing about a 6-day supply of these cells.
• Lymphocytes are mostly stored in the various lymphoid tissues, except for a small number temporarily being transported in the blood.
• Megakaryocytes are formed in the bone marrow, fragment into platelets (or thrombocytes), and pass into the blood.
• Platelets are important in the initiation of blood clotting.

Life Span of White Blood Cells

• Granulocytes circulate in the blood for 4 to 8 hours and survive another 4 to 5 days in tissues where they are needed.
• The life span of granulocytes is shortened to a few hours during serious tissue infection.
• Monocytes have a short transit time of 10 to 20 hours in the blood, before wandering through capillary membranes into the tissues.
• Once in the tissues, monocytes swell to become tissue macrophages and can live for months while performing phagocytic functions.
• Tissue macrophages are the basis of the tissue macrophage system, which provides continuing defense against infection.
• Lymphocytes enter the circulatory system continually and pass out of the blood back into the tissues.
• Lymphocytes re-enter the lymph and return to the blood, with continual circulation and life spans of weeks or months.
• Platelets in the blood are replaced about once every 10 days, with about 30,000 platelets formed each day for each microliter of blood.

Neutrophils and Macrophages Defend Against Infections

• Neutrophils and tissue macrophages attack and destroy invading bacteria, viruses, and other harmful agents.
• Neutrophils are mature cells that can attack and destroy bacteria, even in the circulating blood.
• Tissue macrophages begin life as blood monocytes, which are immature cells in the blood with little ability to fight infectious agents.
• Once monocytes enter the tissues, they swell to as great as 60 to 80 micrometers and become macrophages.
• Macrophages are capable of combating disease agents in the tissues.

White Blood Cells Enter the Tissue Spaces by Diapedesis

• Neutrophils and monocytes squeeze through gaps between endothelial cells of blood capillaries and postcapillary venules by diapedesis.
• A small portion of the cell slides through the gap at a time to fit.

White Blood Cells Move Through Tissue Spaces by Ameboid Motion

• Neutrophils and macrophages move through the tissues by ameboid motion at velocities as great as 40 µm/min.

White Blood Cells Are Attracted to Inflamed Tissue Areas by Chemotaxis

• Chemical substances in the tissues cause neutrophils and macrophages to move toward the chemical source.
• This phenomenon is known as chemotaxis.
• When a tissue becomes inflamed, at least a dozen products, include toxins, degenerative products, complement complex reaction products, and plasma clotting reaction products, can cause chemotaxis toward the inflamed area.
• Chemotaxis depends on the concentration gradient of the chemotactic substance.
• Chemotaxis is effective up to 100 micrometers away from an inflamed tissue.
• The chemotactic signal can easily move WBCs from the capillaries into the inflamed area.

Phagocytosis

• Phagocytosis is a major function of the neutrophils and macrophages, involving cellular ingestion of the offending agent.
• Natural structures in the tissues have smooth surfaces that resist phagocytosis, while rough surfaces increase the likelihood of phagocytosis.
• Natural substances of the body have protective protein coats that repel the phagocytes, while dead tissues and foreign particles have no protective coats.
• The immune system develops antibodies against infectious agents, which adhere to bacterial membranes and make the bacteria especially susceptible to phagocytosis.
• The antibody molecule combines with the C3 product of the complement cascade, which attaches to receptors on the phagocyte membrane, initiating phagocytosis.
• Opsonization is the process whereby a pathogen is selected for phagocytosis and destruction.

Phagocytosis by Neutrophils

• Neutrophils entering the tissues are mature cells that can begin phagocytosis immediately.
• The neutrophil first attaches itself to the particle, projects pseudopodia in all directions, and fuses them.
• An enclosed chamber forms containing the phagocytized particle.
• The chamber invaginates to the inside of the cytoplasmic cavity, breaks away to form a free-floating phagocytic vesicle (phagosome).
• A single neutrophil can usually phagocytize 3 to 20 bacteria before becoming inactivated and dying.

Phagocytosis by Macrophages

• Macrophages are the end-stage product of monocytes that enter the tissues from the blood.
• When activated by the immune system, macrophages can phagocytize about 100 bacteria.
• Macrophages can engulf even whole RBCs or malarial parasites, whereas neutrophils are not capable of phagocytizing larger particles.
• Macrophages can extrude the residual products and function for many more months after digesting particles.

Once Phagocytized, Most Particles Are Digested by Intracellular Enzymes

• Lysosomes and other cytoplasmic granules in the neutrophil or macrophage fuse with the phagocytic vesicle, dumping digestive enzymes and bactericidal agents into the vesicle.
• The phagocytic vesicle becomes a digestive vesicle, and digestion of the phagocytized particle begins immediately.
• Both neutrophils and macrophages contain an abundance of lysosomes filled with proteolytic enzymes for digesting protein matter.
• Macrophages contain lipases, which digest lipid membranes possessed by some bacteria, such as the tuberculosis bacillus.

Neutrophils and Macrophages Can Kill Bacteria

• Neutrophils and macrophages contain bactericidal agents that kill most bacteria, even when the lysosomal enzymes fail to digest them.
• The killing effect results from oxidizing agents formed by enzymes in the membrane of the phagosome or by a special organelle called the peroxisome.
• Oxidizing agents include superoxide (O2¯), hydrogen peroxide (H2O2), and hydroxyl ions (OH¯).
• Myeloperoxidase catalyzes the reaction between H2O2 and chloride ions to form hypochlorite, which is exceedingly bactericidal.
• Some bacteria, notably the tuberculosis bacillus, have coats that resist lysosomal digestion and resist the effects of the neutrophils and macrophages.

Monocyte-Macrophage Cell System (Reticuloendothelial System)

• After entering the tissues and becoming macrophages, a large portion of monocytes becomes attached to the tissues and remains attached for months or even years.
• They can phagocytize large quantities of bacteria, viruses, necrotic tissue, or other foreign particles in the tissue.
• Appropriately stimulated macrophages can break away from their attachments and become mobile macrophages that respond to stimuli related to the inflammatory process.
• The body thus has a widespread monocyte-macrophage system in virtually all tissue areas.
• The reticuloendothelial system consists of monocytes, mobile macrophages, fixed tissue macrophages, and specialized endothelial cells in the bone marrow, spleen, and lymph nodes.
• All or almost all cells in the reticuloendothelial system originate from monocytic stem cells.

Tissue Macrophages in Skin and Subcutaneous Tissues (Histiocytes)

• The skin is mainly impregnable to infectious agents.
• When infection begins in a subcutaneous tissue, local tissue macrophages can divide and form still more macrophages to attack and destroy the infectious agents.

Macrophages in Lymph Nodes

• No particulate matter that enters the tissues can be absorbed directly through the capillary membranes into the blood.
• If the particles are not destroyed locally, they enter the lymph and flow to the lymph nodes, and are trapped in the nodes in a meshwork of sinuses lined by tissue macrophages.
• Large numbers of macrophages line the lymph sinuses, phagocytize particles, and prevent general dissemination throughout the body.

Alveolar Macrophages in Lungs

• The route where by invading organisms frequently enter the body is through the lungs.
• Large numbers of tissue macrophages are present as integral components of the alveolar walls.
• They can phagocytize particles that become entrapped in the alveoli, digest digestible particles, and release the digestive products into the lymph.
• If the particle is not digestible, the macrophages form a giant cell capsule around the particle until it can be dissolved.

Macrophages (Kupffer Cells) in Liver Sinusoids

• The route whereby bacteria invade the body is through the gastrointestinal tract.
• Large numbers of bacteria from ingested food constantly pass through the gastrointestinal mucosa into the portal blood.
• The liver sinusoids are lined with tissue macrophages called Kupffer cells.
• Kupffer cells form an effective particulate filtration system that almost eliminates bacteria from the gastrointestinal tract passing from the portal blood into the general systemic circulation.

Macrophages of Spleen and Bone Marrow

• If an invading organism enters the general circulation, other lines of defense from the tissue macrophage system, especially by macrophages of the spleen and bone marrow exist.
• Macrophages become entrapped by the reticular meshwork of the spleen and bone marrow, and phagocytize foreign particles.

Inflammation: Role of Neutrophils and Macrophages

• Tissue injury results in the release of multiple substances by the injured tissues and causes dramatic secondary changes in the surrounding uninjured tissues.
• This entire complex of tissue changes is called inflammation.

Inflammation

• Inflammation is characterized by:
• Vasodilation of local blood vessels, with consequent increased local blood flow.
• Increased permeability of the capillaries, allowing leakage of large quantities of fluid into the interstitial spaces.
• Clotting of the fluid in the interstitial spaces because of increased amounts of fibrinogen and other proteins leaking from the capillaries.
• Migration of large numbers of granulocytes and monocytes into the tissue.
• Swelling of the tissue cells.
• Some of the tissue products that cause these reactions are histamine, bradykinin, serotonin, prostaglandins, complement system reaction products, blood clotting system reaction products, and lymphokines released by sensitized T cells.
• Macrophages are strongly activated and devour the destroyed tissues.

Walling-Off Effect of Inflammation

• Inflammation walls off the area of injury from the remaining tissues.
• The tissue spaces and lymphatics in the inflamed area are blocked by fibrinogen clots so that fluid barely flows through the spaces.
• The walling-off process delays the spread of bacteria or toxic products.
• The intensity of the inflammatory process is proportional to the degree of tissue injury.
• Staphylococci release extremely lethal cellular toxins, resulting in inflammation that develops more rapidly than the staphylococci spread.
• Streptococci do not cause such intense local tissue destruction, allowing the walling-off process to develop slowly over many hours, while many streptococci reproduce and migrate.

Macrophage and Neutrophil Responses During Inflammation

• Already present macrophages in tissues begin their phagocytic actions.
• The first effect of infection and inflammation is rapid enlargement of each of these cells.
• Many sessile macrophages break loose and become mobile to form the first line of defense against infection during the first hour or so.

Neutrophil Invasion of the Inflamed Area Is a Second Line of Defense

• Large numbers of neutrophils invade the inflamed area from the blood within the first hour or so after inflammation begins.
• This invasion is caused by inflammatory cytokines and other biochemical products from the inflamed tissues.
• The inflammatory cytokines cause increased expression of adhesion molecules causing neutrophils to stick to the capillary and venule walls in the inflamed area (margination).
• Inflammatory cytokines cause intercellular attachments between endothelial cells to loosen, allowing openings for neutrophils to crawl through by diapedesis.
• Inflammatory cytokines cause chemotaxis of the neutrophils toward the injured tissues.
• It is called extravasation when neutrophils or other substances translocation through the capillaries into the tissues surrounding them.
• The specific passage of blood cells through the intact walls of the capillaries is called diapedesis.
• Within several hours after tissue damage begins, the area becomes well supplied with neutrophils.
• Blood neutrophils are mature cells ready to kill bacteria and remove foreign matter.

Acute Increase in the Number of Neutrophils in Blood (Neutrophilia)

• The number of neutrophils in the blood increases fourfold to fivefold within a few hours after the onset of acute severe inflammation, called neutrophilia.
• Neutrophilia is caused by products of inflammation that enter the blood stream, are transported to the bone marrow, and act there on the stored neutrophils of the marrow to mobilize these into the circulating blood.

Second Macrophage Invasion Into the Inflamed Tissue Is a Third Line of Defense

• Monocytes from the blood enter the inflamed tissue and enlarge to become macrophages.
• The number of monocytes in the circulating blood is low, and the storage pool of monocytes in the bone marrow is much less than that of neutrophils.
• The buildup of macrophages in the inflamed tissue area is slower than that of neutrophils.
• Monocytes are still immature cells after invading the inflamed tissue, requiring 8 hours or more to swell to much larger sizes and develop tremendous quantities of lysosomes.
• After several days to several weeks, macrophages dominate the phagocytic cells of the inflamed area because of increased bone marrow production of new monocytes.
• Macrophages can phagocytize more bacteria, larger particles, neutrophils, and necrotic tissue than neutrophils.
• Macrophages play an important role in initiating development of antibodies.

Increased Production of Granulocytes and Monocytes by Bone Marrow Is a Fourth Line of Defense

• Granulocytes and monocytes by the bone marrow increase greatly.
• Stimulation of the granulocytic and monocytic progenitor cells of the marrow results in increased WBC production.
• It takes 3 to 4 days before newly formed granulocytes and monocytes reach the stage of leaving the bone marrow.
• The bone marrow produces these cells in large quantities at a rate 20 to 50 times normal

Feedback Control of Macrophage and Neutrophil Responses

• Tumor necrosis factor (TNF), interleukin-1 (IL-1), granulocyte-monocyte colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF); and (5) monocyte colony-stimulating factor (M-CSF) play dominant roles in control of the macrophage response to inflammation.
• These factors are formed by activated macrophage cells in the inflamed tissues and in smaller quantities by other inflamed tissue cells.
• The cause of increased production of granulocytes and monocytes by the bone marrow is mainly the three colony-stimulating factors.
• GM-CSF stimulates both granulocyte and monocyte production; G-CSF and M-CSF stimulate granulocyte and monocyte production, respectively.
• This combination of TNF, IL-1, and colony-stimulating factors provides a powerful feedback mechanism that begins with tissue inflammation and proceeds to formation of large numbers of defensive WBCs that help remove the cause of the inflammation.

Formation of Pus

• Neutrophils and macrophages engulf large numbers of bacteria and necrotic tissue, and essentially all neutrophils and many macrophages die.
• A cavity is excavated in the inflamed tissues containing necrotic tissue, dead neutrophils, dead macrophages, and tissue fluid. This mixture is known as pus.
• After the infection has been suppressed, the dead cells and necrotic tissue in the pus gradually autolyze and absorbed into the surrounding tissues and lymph until most of the evidence of tissue damage is gone.

Eosinophils

• Eosinophils normally constitute about 2% of all the blood leukocytes exhibit chemotaxis, but are weak phagocytes.
• Eosinophils are produced in large numbers in people with parasitic infections, and migrate into tissues diseased by parasites.
• Eosinophils attach themselves to parasites and release substances that kill many of the parasites.
• Eosinophils release hydrolytic enzymes, reactive forms of oxygen, and a larvicidal polypeptide called major basic protein.
• Trichinosis results from invasion of the body's muscles by the Trichinella parasite (pork worm) after a person eats undercooked infested pork.
• Eosinophils collect in tissues in which allergic reactions occur.
• Mast cells and basophils release an eosinophil chemotactic factor causing eosinophils to migrate toward the inflamed allergic tissue.
• Eosinophils detoxify inflammation-inducing substances released by mast cells and basophils and phagocytize and destroy allergen-antibody complexes.

Basophils

• Basophils in the circulating blood are similar to the large tissue mast cells located immediately outside many of the capillaries in the body.
• Mast cells and basophils liberate heparin and histamine into the blood.
• Mast cells and basophils play an important role in allergic reactions because immunoglobulin E (IgE) has a special propensity to become attached to mast cells and basophils.
• When the specific antigen for the specific IgE antibody subsequently reacts with the antibody, the mast cell or basophil releases increased quantities of histamine, bradykinin, serotonin, heparin, leukotrienes, and lysosomal enzymes.
• These substances cause local vascular and tissue reactions that mediate many of the allergic manifestations.

Leukopenia

• Leukopenia is a clinical condition when the bone marrow produces very few WBCs.
• The body is unprotected against many bacteria and other agents that might invade the tissues.
• Ulcers appear in the mouth and colon, or some form of severe respiratory infection might develop.
• Bacteria from the ulcers rapidly invade surrounding tissues and the blood.
• Irradiation or exposure to drugs and chemicals is likely to cause aplasia of the bone marrow.
• Some stem cells, myeloblasts, and hemocytoblasts may remain undestroyed to regenerate the bone marrow.
• A properly treated patient develops enough new bone marrow for blood cell concentrations to return to normal.

Leukemias

• Uncontrolled production of WBCs is caused by cancerous mutation of a myelogenous or lymphogenous cell.
• Such condition causes leukemia, which is usually characterized by greatly increased numbers of abnormal WBCs in the circulating blood.

Types of Leukemia

• Lymphocytic leukemias: caused by cancerous production of lymphoid cells, beginning in the lymph node or other lymphocytic tissue and spreading to other areas of the body.
• Myelogenous leukemia: begins by cancerous production of young myelogenous cells in the bone marrow, then spreads through the body and WBCs are produced in many extramedullary tissues

Forms of Myelogenous Leukemia

• Neutrophilic leukemia
• Eosinophilic leukemia
• Basophilic leukemia
• Monocytic leukemia
• Often, the leukemia cells are bizarre, undifferentiated and not identical to any of the normal WBCs.
• The more undifferentiated the cell, the more acute is the leukemia, often leading to death within a few months if untreated.
• Process can be chronic with more differentiated cells
• Leukemic cells are usually nonfunctional for providing normal protection against infection.

Effects of Leukemia on the Body

• The first effect of leukemia is metastatic growth of leukemic cells in abnormal areas of the body
• The leukemic cells reproduce and invade the surrounding bone, causing pain and tendency for bones to fracture easily.
• Almost all leukemias eventually spread to the spleen, lymph nodes, liver, and other vascular regions.
• Common effects in leukemia are development of infection, severe anemia, and a bleeding tendency caused by thrombocytopenia (lack of platelets).
• Excessive use of metabolic substrates by the growing cancerous cells happens
• Causes the energy of the patient to be greatly depleted.
• Excessive utilization of amino acids causes rapid deterioration of the normal protein tissues of the body.
• After metabolic starvation has continued long enough, this factor alone is sufficient to cause death.