An organism’s awareness of itself and of its environment depends on communication between one part of its body and another. In biology, such internal messaging is accomplished by several different mechanisms. Humans, like all mammals, use mechanisms involving chemistry and mechanisms involving electricity.
As we explore in this chapter, the nervous system is the body’s electrical communications network. It generates and transmits information throughout the body by converting chemical signals into electrical impulses. The nervous system works closely with the endocrine system, which influences almost every cell, organ, and function of your body by releasing hormones from various glands. The nervous system controls when the endocrine system should release or withhold hormones.
In this chapter, we also consider the immune system. Your immune system is all that stands between you and a planet full of invasive microorganisms that regard you, metaphorically speaking, as a large serving of biological molecules that could feed their own processes.
Controlling It All: The Nervous System
The nervous system reaches into every organ and participates one way or another in nearly every physiological reaction. Perceiving the beauty of a flying bird and digesting your breakfast can happen simultaneously, and each action is dependent on the nervous system.
The nervous system has just three jobs to do, and these jobs overlap:
● Sensory input: Specialized neurons called sensory receptors collect information from the entire body, create an impulse, and transmit the impulse to either the spinal cord or brain stem, and then to the brain.
● Integration: The central nervous system (CNS) makes sense of the input it receives from all around the body.
● Motor output: In response to the integration of the sensory input, the peripheral nervous system (PNS) initiates and sends out impulses through nerves to muscles, glands, and other organs capable of the appropriate response.
The following sections get you acquainted with the various neural tissues and give you a glimpse at the inner workings of the CNS and PNS.
Surveying the neural tissues
The nervous system is made up primarily of two categories of cells — neurons and neuroglial cells — that associate very closely in the brain and in tissues called nerves.
Neurons
A neuron is an individual cell and is the basic unit of the nervous system. Neurons are highly specialized for the initiation and transmission of electrical signals (impulses). The neuron is able to, in an instant, receive the outputs (pulses of electrical energy) of many other cells, process this incoming information, and “decide” whether to generate its own signal to be passed on to other neurons, muscles, or gland cells.
Following are the three types of neurons (see Figure 9-1 for an illustration of the first two):
● Sensory neurons: Also called afferent neurons (afferent means "moving toward"), these neurons respond to sensory stimuli (touch, sound, light, and so on), passing the impulses ultimately to the spinal cord and brain.
● Motor neurons: Also called efferent neurons (efferent means "moving away"), these neurons transmit impulses from the brain and spinal cord to effector organs (muscles and glands), triggering responses from these organs (muscle contraction or release of the gland's product).
● Interneurons: Also called association neurons, these cells connect neurons to other neurons within the same region of the brain or spinal cord.
FIGURE 9-1: Motor neuron and sensory neuron, structure and path of impulses.
Neurons in different parts of the nervous system perform diverse functions and, therefore, vary in shape, size, and electrochemical properties. However, neurons have a special cellular anatomy adapted to the quick transmission of an electrical charge. All neurons have these same three parts, all enclosed within their cell membrane:
● Cell body: The body of a neuron is similar to a generic cell. It contains the nucleus, mitochondria, and other organelles.
● Dendrites: The dendrites are extensions that branch from one end of the cell body. They receive information from other neurons and send impulses in the direction of the cell body.
● Axon: The axon is a cable-like projection located on the opposite end of the cell body from the dendrite. Extending many times (tens, hundreds, or even tens of thousands of times) the cell body's diameter in length, the axon carries the impulses away from the cell body and toward the next neuron in the chain. (Think of electrical transmission wires.)
Fully differentiated neurons don’t typically divide and may live for years, or even the whole lifetime of an organism.
Neuroglial cells
Various types of cells, collectively called neuroglial cells (or just glial cells; glia means “glue”), support neurons in various ways, including physically holding them in place and supplying them with nutrients. They protect neurons from pathogens and remove dead neurons. Certain types of glial cells generate myelin, a fatty substance that wraps around axons and provides electrical insulation that allows the axons to transmit action potentials much more rapidly and efficiently. Scientists estimate that in the human brain, the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.
Nerves
A nerve is a bundle of peripheral axons. An individual axon plus its myelin sheath is called a nerve fiber. Nerves provide a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the PNS. Nerve fibers can be of two types: motor, which send impulses away from the CNS, or sensory, which send impulses toward the CNS.
Ganglia and plexuses
A ganglion (plural, ganglia) is literally a bundle of nerves. Well, almost literally. A ganglion is an aggregation of neuron cell bodies. Ganglia provide relay points and intermediary connections among the body’s neurological structures, especially between the CNS and the PNS.
Ganglia may be connected to form a chain. For example, the sympathetic nervous system contains a chain of ganglia referred to as the paravertebral ganglia or the sympathetic chain of ganglia.
Plexus is a general term for a network of anatomical structures, such as lymphatic vessels, nerves, or veins. (The term comes from the Latin plectere meaning “to braid.”) A neural plexus is a network of intersecting nerves. The solar plexus serves the internal organs. The cervical plexus serves the head, neck, and shoulders. The brachial plexus serves the chest, shoulders, arms, and hands. The lumbar, sacral, and coccygeal plexuses serve the lower body.
Traveling the integrated networks
The nervous system comprises two physically separate but functionally integrated networks of nervous tissue. Working together, these networks perceive and respond to internal and external stimuli to maintain homeostasis and simultaneously move the genetic development program forward. The following sections take a closer look at the central and peripheral nervous systems.
Central nervous system
The central nervous system (CNS), which consists of the brain and spinal cord, is the largest part of the nervous system. It integrates the information it receives from the sensory receptors and coordinates the activity of all parts of the body.
Both the brain and spinal cord are masses of neural tissue protected within bony structures (the skull and the vertebral column, respectively) and layers of membranes and specialized fluids, reflecting their prime importance to the continuation of the organism’s life.
The brain and spinal cord are made up mainly of two types of tissue, called gray matter and white matter. Gray matter consists of unmyelinated neurons, neuron cell bodies, and neuroglial cells. White matter is made up of neuroglial cells and the myelinated axons extending from the neuron cell bodies in the gray matter. (See the “Surveying the neural tissues” section, earlier in this chapter, for more on neurons and neuroglial cells.) The myelin has a high fat content, which results in white matter’s white color.
In the brain, the gray matter forms a thin layer on the outside (the cortex). The white matter is beneath and makes up the brain’s big data lines, carrying information around the brain. In the spinal cord, the tissue is arranged in a long cylinder; the gray matter forms the inner layer, and the white matter forms the outer layer.
The spinal cord extends from the bottom of the brain stem down the vertebral column within a cylindrical tubular opening created by the vertebrae and three tough membranes with cushioning fluid between them.
Peripheral nervous system
The peripheral nervous system (PNS) consists of the nerves and ganglia outside of the brain and spinal cord. Unlike the CNS, the PNS isn’t protected by bone or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. Structures of the PNS include the following:
● Cranial nerves: Twelve pairs of nerves that emerge directly from the brain and brain stem. Each pair is dedicated to particular functions — some bring information from the sense organs to the brain, others control muscles, but most have both sensory and motor functions. Some are connected to glands or internal organs such as the heart and lungs. For example, the longest of the cranial nerves, called the vagus nerves, pass through the neck and chest into the abdomen. They relay sensory impulses from part of the ear, tongue, larynx, and pharynx; relay motor impulses to the vocal cords; and relay motor and secretory impulses to some abdominal and thoracic organs.
● Spinal nerves: Thirty-one pairs of nerves that emerge from the spinal cord. Each contains thousands of afferent (sensory) and efferent (motor) fibers.
● Sensory nerve fibers: Nerve fibers all over the body that send impulses to the CNS via the cranial nerves and spinal nerves.
● Motor nerve fibers: Nerve fibers that connect to muscles and glands and send impulses from the CNS via the cranial and spinal nerves.
The PNS is further divided into the somatic system and the autonomic system.
● The somatic nervous system regulates activities that are under conscious control. Its sensory fibers receive impulses from receptors. Its motor fibers transmit impulses from the CNS to the (voluntary) skeletal muscles to coordinate body movements.
● The motor fibers of the autonomic system transmit impulses from the CNS to the glands, heart, and smooth (involuntary) organ muscles. The autonomic system controls internal organ functions that are involuntary and that happen subconsciously, such as breathing, heartbeat, and digestion.
This system is made up of the following:
• Sympathetic nervous system: Nerves originate in the thoracic and lumbar regions of the spinal cord. The sympathetic nervous system responds to stress and is responsible for the increase of your heartbeat and blood pressure, among other physiological changes, along with the sense of excitement you may feel due to the increase of adrenaline in your system.
• Parasympathetic nervous system: Nerves originate in the brain stem and sacral portion of the spinal cord. The parasympathetic nervous system is evident when you rest or feel relaxed and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and urinary systems. The parasympathetic nervous system is known as the housekeeping system because it maintains your normal functioning when you're not under stress.
• Enteric nervous system: This system manages every aspect of digestion, from the esophagus to the stomach to the small intestine to the colon.
Sending Messages Chemical-Style with the Endocrine System
In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth. The nervous system, described in the earlier section “Controlling It All: The Nervous System,” manages faster processes such as breathing and body movement. The following sections explain the functions of hormones and the glands they come from.
Homing in on hormones
A hormone is an endogenous substance (one that’s produced within the body) that has its effects in specific target cells. Hormones are many and varied in their source, chemical nature, target tissues, and effects. However, they’re characterized by the fact that they’re synthesized in one place (gland or cell) and they travel via the blood until they reach their target cell. Hormones are bound by specific receptors in their target cells. The binding of the hormone on the receptor induces a response within the cell.
Among glands, the pituitary, thyroid, and adrenal glands are most well-known. These organs have no significant function other than to produce hormones. However, a number of other endocrine tissues and hormones, though less well-known, are just as important in controlling vital bodily functions. In fact, all the tissue in your body is, in some way, an endocrine tissue.
Hormone chemistry
Chemically, hormones fall into four types:
● Lipid hormones: Lipid and phospholipid hormones are derived from fatty acids. The best-known lipid hormones are the steroids, such as estrogen, progesterone, testosterone, aldosterone, and cortisol (which is synthesized from cholesterol). Another group of lipid hormones is called prostaglandins.
● Peptide hormones: Peptides are relatively short chains of amino acids. Peptide hormones include antidiuretic hormone (ADH), thyrotropin-releasing hormone (TRH), and oxytocin.
Other hormones are proteins (chains of peptides), such as insulin, growth hormone, and prolactin.
● Glycoprotein hormones: More complex protein hormones bear carbohydrate side-chains and are called glycoprotein hormones. These include follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH).
● Amine hormones: Amine hormones are derivatives of the amino acids tyrosine and tryptophan. Examples are thyroxine, epinephrine, and norepinephrine.
Hormone sources
At one time, and not so long ago, by definition a hormone was produced in an endocrine gland (and an endocrine gland was a structure that produced one or more hormones). But as biologists have discovered and described more and more hormone substances and forms, they’ve expanded the definition to include similar, sometimes identical, substances that have a similar mechanism of action, wherever they’re produced. Check out all the sources of hormones:
● Endocrine glands: An endocrine gland is an organ that synthesizes a hormone. It does so within a specialized cell type — the anterior pituitary gland, for instance, has cells that specialize in the production of such hormones as adrenocorticotropic hormone (ACTH), growth hormone, and TSH. Specialized cells within the thymus synthesize hormones that control the maturation of immune cells.
● Various organs: A number of organs not usually included within the endocrine system by anatomists and physiologists have cells and tissues specialized for the production of hormones. For example:
• While part of the pancreas is busy secreting enzymes for the digestion of food, other specialized cells of the pancreas produce insulin, and others produce glucagon.
• The stomach and intestines, too, synthesize and release hormones that control both physical and chemical aspects of digestion.
• Specialized cells in the ovaries and testes transform cholesterol molecules into molecules of estrogen and testosterone, respectively.
• Even the heart produces hormones, the secretion of which has an immediate strong effect on blood volume (fluid balance).
● Neurons: Neurons make hormones that are neurotransmitters. Or, to look at it another way, hormones are substances that transmit physiologically significant messages with considerable subtlety; therefore, not surprisingly, they're synthesized and released in different physiological messaging contexts — the transmission of nerve impulses across a synapse, for example. The only difference between epinephrine synthesized in the adrenal glands and epinephrine synthesized in nerve cells is the distance the molecules travel to their target site.
Hormone receptors
Hormones exit their cell of origin via exocytosis, which involves a sac or vesicle enveloping the substance and moving it across the cell membrane, or another means of membrane transport. A secreted hormone molecule goes directly into the blood and circulates until it enters a cell or binds with its specific receptor on the cell membrane, where through second messengers its effects may be profound.
The presence of a specific hormone receptor makes that cell a target for the hormone. Without the target receptor, the hormone has no effect.
The receptor may be on or embedded in the cell membrane, as is typically the case for peptide hormones. The hormone molecule, called the first messenger, is taken into the cell via active transport, stimulating the production of a compound, cyclic AMP (cyclic adenosine monophosphate), called the second messenger, thus causing the target cell to produce the necessary enzymes — that is, to induce the expression of a certain gene. (We describe the process of active transport in Chapter 2.)
A steroid hormone molecule doesn’t require a cell-membrane receptor or active transport. As a lipid, it enters a cell by diffusing through the membrane. After it’s inside the cell, it binds with target receptor molecules in the cytoplasm. The receptor-hormone complex moves into the nucleus, where it activates the expression of the gene for a needed enzyme.
Grouping the glands
In general, a gland is a structure that synthesizes a product that’s exported from the cells. Endocrine (ductless) glands export their products (hormones) via the bloodstream to their target cells in anatomically distant organs. The following sections give you the lowdown on the endocrine glands.
The taskmasters: The hypothalamus and pituitary
The hypothalamus can be considered the location where the nervous system and the endocrine system meet. It’s sometimes called the master gland because it ultimately controls the functioning of other glands, acting through the pituitary.
The hypothalamus contains special cells that act as sensors that “analyze” the composition of the blood as it circulates through. It also contains other specialized cells that generate messengers (hormones) in response to the analysis. Tight pairing between these two types of cells is essential for homeostasis.
The pituitary gland has two parts, called the anterior pituitary and posterior pituitary, that have different relationships with the hypothalamus.
● The anterior pituitary gland secretes many hormones, including melanocyte-stimulating hormone (MSH). This hormone directly stimulates melanocytes to produce melanin pigment, which protects the skin from sunlight damage. This gland also secretes prolactin, which is responsible for the increase in size of the lactiferous glands in the breast and the production of milk. It also secretes the gonadotropic hormones FSH and LH, which target the ovaries and testicles, and ACTH, which targets the cortex of the adrenal glands. The function of these pituitary hormones is to stimulate the release of other hormones from their target glands.
● The posterior pituitary gland, which is directly connected to the hypothalamus, releases hormones that are actually synthesized in the nerve cell bodies of the hypothalamus and travel down the axons that end in the posterior pituitary.
One such hormone is ADH. When the blood's fluid volume falls below the ideal range, the hypothalamus produces ADH, which travels down the axons into the posterior pituitary gland. Released by the pituitary into the blood, ADH reaches its target kidney cells. Via active transport, ADH enters the cells of the tubules and alters the cells' metabolism so more water is removed from the urine that the kidney produces and is added to the blood.
Controlling metabolism
Two relatively small organs exert a major effect on the availability of energy for physiological processes:
● The thyroid gland: Straddling your trachea (windpipe) and looking somewhat like a butterfly, your thyroid gland secretes hormones that affect almost every physiological process in the body. The thyroid hormones
• Regulate the rate at which cells metabolize and respire (use oxygen and release carbon dioxide).
• Increase the rate at which cells use glucose and stimulate the breakdown of glycogen (the storage form of glucose) into individual glucose molecules so the blood level of glucose increases.
• Help maintain body temperature by increasing or decreasing metabolic rate.
• Regulate growth and differentiation of tissues in children and teens.
• Increase the amount of certain enzymes in the mitochondria that are involved in oxidative reactions.
• Influence the metabolic rate of proteins, fats, carbohydrates, vitamins, minerals, and water.
• Stimulate mental processes.
● The adrenal gland: Found near the kidneys, each adrenal gland consists of two parts, the cortex and the medulla.
• The adrenal cortex secretes corticosteroids, which include mineralocorticoids, glucocorticoids, and gonadocorticoids.
One of the most important mineralocorticoids is aldosterone, which is responsible for regulating the concentration of electrolytes, such as potassium (K+), sodium (Na+), and chloride (Cl-) ions. This regulation keeps the blood's salt and mineral content within the ranges required for homeostasis. The main glucocorticoid hormone, cortisol, regulates the metabolism of proteins, fats, and carbohydrates. Your body releases cortisol when you're stressed emotionally, physically, or environmentally.
• The adrenal medulla, which developed from the same tissues as the sympathetic nervous system, is responsible for regulating a class of hormones called the catecholamines, of which epinephrine and norepinephrine are the best known. Epinephrine, also called adrenaline, initiates the adrenaline rush of the fight-or-flight response.
Getting the gonads going
Your gonads — ovaries if you’re female or testes if you’re male — produce and secrete the steroid sex hormones throughout your lifetime at different levels. Their production increases at puberty and normally decreases as you age. Here’s the scoop on the different sex hormones:
● Estrogen: In women, the increased production of estrogen at puberty is responsible for initiating the development of the secondary sex characteristics, such as the enlargement of the breasts.
● Progesterone: This hormone works to prepare the uterus for implantation of a pre-embryo by causing changes in uterine secretions and in storing nutrients in the uterus's lining. Progesterone also contributes to breast development.
● Testosterone: This hormone causes the development of secondary sex characteristics in males.
Defending Your "Self" with the Immune System
Your immune system consists of a variety of components: the lymphatic system, a body-wide network of vessels and organs through which flows an important body fluid called lymph; a variety of very peculiar cell types; and several types of biological molecules, some of them just as peculiar. These components all have specialized functions for attacking and eliminating microbial invaders:
● Confronting marauders: Whether you're well or ill, your immune system is always alert and active. That's why none of the bacteria, fungi, parasites, and viruses that are present by the uncountable millions in the air you breathe, on surfaces you touch, and on your food, are eating you.
● Stopping renegades: The second major function of the immune system is to recognize and destroy cells of your own body that have gone rogue — potential seeds for cancerous growth.
Loving Your Lymphatic System
The lymphatic system plays a crucial role in circulation by draining the fluids that pour out into the extracellular space during capillary exchange and returning them to the blood. The lymphatic system is more than a drainage network, though. It removes toxins, helps transport fats, and stabilizes blood volume despite environmental stresses. Possibly its most interesting functions are those related to its role in the immune system, fighting biological invaders.
Lymphing along
Interstitial (extracellular) fluid is a watery solution containing oxygen, ions, glucose and other nutrients, proteins, hormones, and so on. The total volume of interstitial fluid, which is found in between cells, is about 2 to 4 pints (1 to 2 liters) at any given moment.
Like plasma, interstitial fluid is continuously flowing: The pressure of the heartbeat pushes this watery solution across the capillary cell membrane and out into the interstitial space, a total of about 50 pints (24 liters) per day. Most of it is reabsorbed into the blood at the venous end of the capillaries. The rest goes on a detour through the lymphatic system. The fluid coursing through the lymphatic system is called lymph. After passing through the lymphatic system, the fluid rejoins the circulatory system through two large veins and becomes plasma again.
Structures of the lymphatic system
The structures of the lymphatic system resemble those of other organs and systems that function to move fluids around. The lymphatic system has its own tubes, pipes, connectors, reservoirs, and filters. It lacks its own pumping organ but, like venous circulation, makes use of skeletal muscle action for this purpose.
LYMPHATIC VESSELS
The lymphatic vessels are the tubes that carry lymph. They form a network very similar to the venous system. You can even think of the lymphatic system as an alternative venous system, because the lymph that the vessels transport comes out of the arterial blood and is delivered back into the venous blood. Like the venous system, the lymphatic system’s vessels start small (the lymph capillaries) and get larger (the lymphatic vessels), and even larger (the lymphatic ducts). Like veins, lymphatic vessels rely on skeletal muscle action and valves to keep the fluid moving in the right direction. The structure of the lymphatic vessel wall is similar to that of the veins, but thinner. Lymph vessels are distributed through the body, more or less alongside the blood vessels.
LYMPHATIC DUCTS
The largest of the lymphatic vessels, the lymphatic ducts, drain into two large veins. The right lymphatic duct, located on the right side of your neck near your right clavicle, drains lymph from the right arm and the right half of the body above the diaphragm into the right subclavian vein. The thoracic duct, also called the left lymphatic duct, which runs through the middle of your thorax, drains lymph from everywhere else into the left subclavian vein.
LYMPH NODES
Lymph nodes are bean-shaped structures located along the lymph vessels. Dense clusters of lymph nodes are found in the mouth, pharynx, armpit, groin, all through the digestive system, and other locations. Each lymph node is covered by a fibrous connective tissue capsule. Afferent lymphatic vessels cross the capsule on the convex side, bringing lymph into the node. The node’s efferent vessel, which carries the filtered lymph out of the node, emerges from the indentation on the concave side of the capsule, called the hilus (as in the kidney).
On the inner side, the capsule sends numerous extensions that divide the node internally into structures called nodules. A nodule is filled with a meshlike network of fibers to which lymphocytes and macrophages (another immune system cell type) adhere. As the lymph flows through the node, pathogens, cancerous cells, and other matter in the lymph are engulfed and destroyed by macrophages or marked for destruction by B lymphocytes. The cleaned-up lymph travels toward the venous system in the efferent vessels.
The lymph nodes also provide a safe and nurturing environment for developing lymphocytes. (See the “Lymphocytes” section, later in the chapter.)
THE SPLENDID SPLEEN
The spleen is a solid organ, located to the left of and slightly posterior to the stomach. It’s roughly oval in shape, normally measuring about 1 x 3 x 5 inches (3 x 8 x 13 centimeters) and weighing about 8 ounces (23 grams). Essentially, its structure is that of a really large lymph node, and it filters blood in much the same way the lymph nodes filter lymph, removing pathogen cells along with exhausted RBCs and many kinds of foreign matter.
The spleen’s structure is similar to that of other organs, like the kidney, the liver, and the lymph nodes. The spleen is enveloped by a fibrous capsule. The spleen has a hilus, a spot where several different vessels cross the capsule. The spleen’s hilus contains the splenic artery, the splenic vein, and an efferent lymph vessel, a similar configuration to the lymph node. Note that the spleen has no role in filtering lymph and no afferent lymph vessels.
Inside, the spleen is divided into functional subunits by outgrowths of the capsule’s fibrous tissue. Within each subunit, an arteriole is surrounded by material called white pulp — lymphoid tissue that contains lymphocyte production centers. Farther toward the outer edges of each compartment, similar masses called red pulp surround the arteriole. The red pulp is a network of channels filled with blood, where most of the filtration occurs. (It’s also the major site of destruction of deteriorating RBCs and the recycling of their hemoglobin.) Both white pulp and red pulp contain leukocytes that remove foreign material and initiate an antibody-producing process.
THE THYMUS GLAND
The thymus gland overlies the heart and straddles the trachea, sitting just posterior to the sternum. It produces thymosin, a hormone that stimulates the differentiation and maturation of T cells. The thymus is relatively large in childhood; it decreases in size with age.
Identifying immune system cells
Immune system cells are special in many ways. In shape and size, they’re far from the compact epithelial or muscle cell types. Immune system cells have about a dozen distinctive shapes and many different sizes, and some have the ability to transform themselves into other, even weirder forms and to multiply extremely rapidly. The next sections give you a closer look at specific types of immune system cells.
Looking at leukocytes
Immune system cells are called leukocytes (white cells), because they appear white in color under a microscope. (That’s white as in pus.) Although all blood cells, red and white, develop from hematopoietic stem cells in the red marrow, the leukocytes contain no hemoglobin and no iron. Unlike RBCs, all leukocytes retain their nuclei, organelles, and cytoplasm through their life cycle. Many fewer leukocytes are produced than RBCs, by a factor of around 700.
Also called white blood cells (WBCs), leukocytes are present everywhere and function at all times. You notice their presence in the acute phase of certain diseases — the immune response, not the invader directly, produces the well-known symptoms of flu. They function not only in the blood (really, in the plasma) but also in the interstitial fluid and the lymph. They’re never far from a site of injury or infection because they’re everywhere. When a splinter pierces your finger, a contingent of local WBCs arrives at the site instantaneously.
Sometimes it’s difficult to remember that these bizarre warrior cells with their amazing superpowers are your cells, just like your skin cells and your blood cells. Discussing them is difficult without resorting to language that makes them seem like a quasi-military force from outside your body. These cells are acutely aware (metaphorically speaking) that “they” are “self” (you). In fact, that’s the primary distinction that matters to them: self or nonself. The overarching mission of a leukocyte is to protect self from other biotic (living) nonself, destroying the invaders where possible and necessary and establishing more or less mutualistic relationships with other life forms where possible and necessary. It’s hard not to picture a disciplined army; but remember it’s a metaphor.
Lymphocytes
The lymphocytes are one group of leukocytes (WBCs). The group includes the B cells and T cells (small, agranular lymphocytes), as well as NK cells (large granular lymphocytes). To simplify: Activated B cells produce antibodies; some T cells destroy antigens; and NK cells attack cancerous cells and cells infected by viruses. The surfaces of lymphocytes are covered with receptors, which are molecules that fit with a specific antigen.
All lymphocytes originate in the red marrow from the same type of hematopoietic stem cell. B lymphocytes and NK cells leave the marrow fully differentiated and enter the circulatory and lymphatic systems. T cells travel to the thymus gland to complete their differentiation in an environment rich in the hormone thymosin. Then they move to the medulla of a lymph node, where they further differentiate into one of several different cell types, each with its own function in the immune response: helper T cells, cytotoxic (cell killing) T cells, or suppressor T cells.
Phagocytizing leukocytes
Several different types of WBCs perform their function in part by processes involving phagocytosis (the digestion of foreign material).
● Neutrophils are the most numerous of the WBCs (40 to 70 percent of the total number) and are continuously present and active in the circulatory and lymphatic systems. Neutrophils squeeze through the capillary walls and into infected tissue, where they phagocytize the invading bacteria. They also function to limit the populations of the beneficial bacterial species in the respiratory and digestive passages.
● Monocytes aren't really stem cells, but they have some functions in common with stem cells — they exist to produce other specialized cells on demand. Monocytes divide to produce two other kinds of immune cells, macrophages and dendritic cells. In a homeostatic state, the monocytes replenish these cells as necessary. In response to inflammation- response-related stimuli, monocytes travel to the site and begin to turn out vast numbers of its daughter cells.
Macrophages (literally, "big eaters") are large phagocytic cells that target antigens and dead self cells. In the early stages of the immune response, macrophages initiate the mass production of other types of WBCs. Dendritic cells take macro- phagy to a whole new level beyond the scope of this book.