The Immune System (CH.43), Osmoregulation, and Excretion (CH. 44)

What is the function of the immune system?

The function of the immune system is to target and destroy pathogens that invade the body. Pathogens are microbes and viruses that cause disease.

How can the immune systems of animals be (most broadly) be split?

The immune system of animals can be split, most broadly, between innate and adaptive immunity.

What is innate immunity?

Innate immunity is a defense system that operates in all animals. This line of defense includes front line defense strategies like barrier defenses, cells specialized in the rapid recognition of pathogens, and chemical defenses.

Where does innate immunity begin in insects?

In insects, innate immunity begins with the chitin exoskeleton-a barrier defense that protects against intruding microorganisms.

What happens when pathogens breach the chitin exoskeleton in insects?

Once this first line of defense is breached, specialized cells circulating in the hemolymph seek and destroy invading pathogens.

What are hemocytes?

Hemocytes are the phagocytic cells that recognize pathogens from the molecules on their cell surface. Once engulfed by a hemocyte, the pathogen is confined to a vacuole that fuses with a lysosome containing enzymes that will destroy it. The enzymatically digested pathogen is then exported out of the phagocytic cell by exocytosis

What are antimicrobial peptides?

Some kinds of hemocytes circulating in the hemolymph secrete short-chain amino acids called antimicrobial peptides.

How do antimicrobial peptides regulate pathogens?

These antimicrobial peptides can perforate the plasma membrane of pathogens and instantly cause them to lyse. Given that bacteria lack a cytoskeleton, puncturing their plasma membrane causes the contents of the cell to spill out.

How do lymph vessels

The immune system of mammals features an elaborate network of lymph vessels that link body tissues to the organs associated with immune function. Intermingled with blood capillaries, the blunt ends of lymphatic vessels pick up excess interstitial fluid bathing body cells along with any pathogens.

How do lymphatic vessels ensure uni-directional flow?

Much like veins, lymphatic vessels are equipped with one-way valves that move fluid uni-directionally by bulk flow toward a vast network of lymph nodes. Within these nodes, the lymph fluid is cleared of pathogens before draining back into the circulatory system.

How do lymphatic vessels clear pathogens from the circulatory system?

Within these nodes, the lymph fluid is cleared of pathogens before draining back into the circulatory system.

Where does the interstitial fluid collected from the top-right half of the body drain?

The interstitial fluid collected from the top-right half of the body eventually drains into the right subclavian vein, while the fluids picked up from the left and bottom half of the body drain into the left subclavian vein.

What are the primary tissues of the immune system?

The primary tissues of the immune system include the bone marrow and thymus; this is where the cells of the adaptive immune system develop and mature.

What are the secondary tissues of the immune system?

The secondary tissues of the immune system include the adenoid gland, tonsils, spleen, Peyer's patch, appendix, and large intestine.

What are the barrier defenses in mammals?

In mammals, barrier defenses include the skin, secretions from glands, and the mucous membranes of the digestive, respiratory, and urogenital tracts.

What is the mucus elevator?

In the respiratory tract, the beating cilia of the columnar epithelial cells move pathogens trapped in the mucus up toward the pharynx where they are swallowed. This mucus elevator prevents pathogens from reaching the alveoli.

How do acidic secretions make certain areas inhospitable to bacteria?

The acidic secretions from sebaceous glands in the dermis, lacrimal glands above the eyes, and salivary glands in the mouth all contribute to making these surfaces inhospitable to pathogenic bacteria. Pathogens that escape the innate defenses of the oral cavity are destroyed by the highly acidic secretions from the gastric glands in the stomach.

What happens when pathogens get past our barrier defenses?

Once breached, a convoy of phagocytic cells begin to phagocytize the intruders.

What are Toll-like receptors?

Receptors on the surface of phagocytic cells, called Toll-like receptors (or TLR), recognize generic carbohydrates, proteins, and lipids that are typically featured on the surface of pathogenic microbes.

What do pathogens do once they're recognized by TLRs?

Once recognized by TLRs, pathogens are phagocytized much like what we saw with the invertebrates. Trapped within vesicles, they are destroyed when the vesicle fuses with a lysosome.

What are leukocytes?

Leukocytes, or white blood cells, are derived from stem cells in the bone marrow.

What do white blood cells include?

White blood cells include neutrophils, monocytes, basophils, eosinophils, and lymphocytes.

Which cells are the phagocytic cells of the immune system?

The neutrophils, and the derivative cells of monocytes-namely the macrophages and dendritic cells-are the phagocytic cells of the innate immune system.

What are eosinophils and basophils?

Eosinophils and basophils also function in innate immunity but are not phagocytic. Both target multicellular pathogens.

What three types of cells do lymphocytes include?

The lymphocytes include three types of cells: B cells, T cells, and natural killer cells

Of B cells, T cells, and natural killer cells, which ones are a part of innate immunity?

Only natural killer cells are part of innate immunity.

What do natural killer cells do?

These killer cells seek and destroy virus-infected and cancer cells.

What are neutrophils?

Neutrophils are the notoriously lethal phagocytic cells of the innate immune system.

Where are neutrophils found?

Neutrophils circulate instead in the blood and only gain access to tissues upon infection

Where are monocytes found?

Monocytes travel to specific body tissues, like the dermis, and differentiate into macrophages.

Why are macrophages large?

Macrophages have phagocytic abilities but are longer lived than neutrophils: in other words, they do not die after engulfing a pathogen and may continue to engulf even more pathogens.

How do macrophages interact with pathogens?

Upon encountering and phagocytizing a pathogen, macrophages secrete signaling molecules called cytokines that cause mast cells to degranulate and release histamines. These local signaling molecules cause nearby blood capillaries to dilate, allowing neutrophils circulating in the blood to gain access to the infected tissues and begin phagocytizing. (The inflammatory response)

During the inflammatory response, why doesn't the flow rate increase in dilated blood vessels?

Despite the increase in vessel diameter, the flow rate of these dilated blood vessels does not increase because of the concomitant increase in blood viscosity triggered by the rise in neutrophils.

In addition to the dermis, where are macrophages found?

In addition to the dermis, we also find resident macrophages in the alveoli of lungs.

What do macrophages do in the alveoli of lungs?

At this location, they engulf pathogens that reach the terminal ends of the respiratory tract

What do macrophages do in the lymph nodes?

Lymph fluid drains into lymph nodes from afferent lymphatic vessels. Macrophages residing permanently in lymph nodes clear the fluid from any pathogens before leaving the node via the efferent lymphatic vessels which drain the fluid into the left and right subclavian veins.

What are dendretic cells? Where are they found?

Dendritic cells function like macrophages. They reside in various tissues throughout the body, but upon encountering and phagocytizing a pathogen they enter lymphatic vessels and travel to lymph nodes where they interact with B cells and T cells to activate the adaptive immune system. Dendritic cells are therefore the link between innate and adaptive immunity.

Are natural killer cells phagocytic?

No; they are non-phagocytic

What do natural killer cells do when they come in contact with a pathogen?

Upon detection of virus-infected or cancer cells, NK cells release molecules, called perforin, that perforate the plasma membrane of the cell.

How does perforin from NK cells perforate plasma membranes of virus-infected cells?

The perforation is achieved by the polymerization of the perforin proteins as they embed themselves in the plasma membrane of targeted cells.

What happens to virus-infected cells after perforins perforate the cells?

Following the perforins, granzymes spill into the pores and destroy the abnormal cell from the inside. Macrophages and dendritic cells respond to the signals released by the NK cells and phagocytize this debris.

What are antimicrobial peptides?

Antimicrobial peptides are short polypeptides that can destroy pathogens, flag them for destruction, or prevent them from replicating.

What are interferons?

Interferons are another form of protein-based chemical defenses produced by virus infected body cells.

How do interferons work?

Interferons travel by diffusion to nearby uninfected cells where they bind to surface proteins and trigger a signal transduction pathway that leads to the production of anti-viral proteins. When the virus infects such cells, the anti-viral proteins block the virus from replicating.

What are lymphocytes?

The cells of the adaptive immune system are called lymphocytes

Where do B and T cells mature?

While all lymphocytes originate in the bone barrow, only B cells remain in the bone marrow to complete maturation; T cells migrate to the thymus to mature.

What are antigens? How do they relate to B and T cells?

B and T cells are designed to recognize specific parts, of specific molecules, on the surface of specific pathogens. These surface molecules, called antigens, can be proteins or polysaccharides

What is the epitope?

B cells and T cells have receptors anchored to their plasma membranes that will bind to just one small part of the antigen. This small part is called the epitope, and it is simply a small cluster of amino acids or sugar molecules.

What are humoral and cell-mediated immunnity?

B cells serve to destroy pathogens circulating in body fluids while T cells target pathogens hiding within body cells. These two distinct battle fronts are called humoral and cell-mediated immunity, respectively.

How are B-cell antigen receptors shaped?

B cell antigen receptors are Y-shaped and composed of 4 polypeptides: two light chains and two heavy chains

How do the variable and constant regions of the B-cell antigen receptor vary?

As the name implies, the sequence of amino acids that make up the constant regions is the same for all B cells; conversely, the variable regions consist of a random sequence of amino acids.

Which part of B-cell antigen receptors do epitopes bind to?

The epitopes of antigens will bind to the terminal ends of the variable regions.

How are T-cell antigen receptors shaped?

T cell antigen receptors are stick-like and composed of just two polypeptides: an alpha chain and beta chain.

Which part of antigen receptors in B and T cells anchors them to the plasma membrane?

The constant region anchors the receptor to the cell membrane.

How do the variable and constant regions of the T-cell antigen receptor vary?

The amino acid sequence of the constant region is invariable-all T cells have identical polypeptides that make up the constant region. But the variable region varies.

How do B and T cell receptors bind to epitopes?

With such vast numbers of different B cells and T cells, some of these antigen receptors will by chance alone fit snuggly to the epitope of an antigen.

How are B and T cells different in how they interact with antigens?

The difference between the two types of lymphocytes is that B cells bind to intact antigens while T cells only respond to antigens anchored on the surface of host cells.

How is the risk of self-reactivity reduced in B and T cells?

The risk of self-reactivity is reduced or eliminated because once B cells and T cells mature, they are tested for self-reactivity. Lymphocytes that react to self-tissues are destroyed.

How does the concentration of lymphocytes change once exposed to an antigen?

While the diversity of lymphocytes is very high, the concentration of each type is kept very low until they recognize and bind to an antigen. This initiates the proliferation phase where their concentrations increase by many orders of magnitude.

What is clonal selection?

The proliferation phase of lymphocytes is called clonal selection.

What do macrophages and dendritic cells do once engulfing a pathogen?

Upon engulfing a pathogen, macrophages and dendritic cells move antigens to the outer surface of their plasma membranes and affix them to a protein complex called major histocompatibility complex (MHC)

Which cells in the body have MHC receptors on their plasma membrane?

All cells in the body, except for red blood cells, have MHC receptors on their plasma membrane.

Which cells in the body have class II MHC receptors on their plasma membrane?

Macrophages, dendritic cells, and B cells have a unique variety called class II MHC. All other body cells have class I MHC.

What happens once the antigen is displayed on class II MHC?

Once displayed on a class II MHC, antigen presentation begins in the lymph nodes to a stream of naïve T-cells with different antigen receptors looking for the one that will bind to an epitope of the displayed antigen.

What happens once a T-cell binds to the epitope on the antigen displayed on the class II MHC?

A helper T cell with the right fit will begin to secrete signaling molecules. These molecules, called cytokines, will trigger the activated helper T cell to proliferate into a million more copies of itself. Activated helper T cells will also initiate the proliferation of B cells that recognize that same antigen. The result of this step is the clonal selection of activated B cells.

What cells do the proliferation stages of B and T cells produce?

The proliferation stage of B cells and T cells produces two types of cells: effector cells and memory cells.

What are plasma cells?

Plasma cells are the effector cells of activated B cells

How are plasma cells?

Plasma cells have an exaggerated endoplasmic reticulum that begins to secrete soluble antigen receptors called antibodies.

How do antibodies interact with pathogens?

Secreted into body fluids, antibodies bind to pathogens, including viruses, bacteria, and toxins, flagging them for destruction.

How do antibodies on pathogens cause it to lyse?

Antibodies on pathogens attract the complement system where the polymerized proteins on the plasma membrane of the pathogen cause it to lyse.

What are cytoxic T cells?

Cytotoxic T cells are the effector cells of activated helper T cells. Cytotoxic T cells destroy virus-infected body cells.

What type of antigens do cytotoxic T cells defend against?

Cytotoxic T cells defend against intracellular pathogens.

How do cytotoxic T cells work? How are the antigens different?

Cytotoxic T cells recognize antigens displayed on infected host cells. Unlike the antigens displayed on the class II MHC molecules of antigen-presenting cells, these antigens are affixed to class I MHC molecules.

Why is the distinction between phagocytic immune cells and body cells important?

The ability to make the distinction between phagocytic immune cells and body cells is crucial: without the distinction, cytotoxic T cells would destroy phagocytic cells.

What happens when cytoxic T cells

Once bound to an antigen-MHC1 complex, cytotoxic T cells work much like natural killer cells: perforin perforates the plasma membrane and granzymes enter into the cytosol and liquefy the cell's enzymes. Phagocytic cells clean up the messes left behind by cytotoxic T cells.

What produces memory cells?

Activated B cells and T cells also produce memory cells that continue to circulate in low

concentrations in the blood until the pathogen is encountered again.

What are the benefits to having memory cells?

The benefit of

memory cells is a rapid response upon re-infection.

What role does the excretory system play?

The role of the excretory system is to eliminate body wastes. For many animals, this organ system also functions in maintaining the osmolarity of body fluids

How is fluid balance between extracellulqr and intracellular fluid compartments achieved?

Fluid balance between the extracellular and intracellular fluid compartments is achieved

by electrolyte homeostasis.

What are electrolytes?

Obtained from ingested foods, electrolytes are minerals that

populate the intracellular and extracellular fluid compartments.

What is the 'extracellular compartment' referring to, in the context of maintaining fluid balance in the body?

The extracellular compartment includes both the interstitial fluid bathing body cells and the intravascular fluid in blood vessels.

What is our principal extracellular electrolyte?

Sodium is our principal and most important extracellular

electrolyte. Other extracellular electrolytes include chloride and calcium ions

What are our principal intracellular electrolytes?

The principal intracellular electrolytes are potassium and magnesium.

How do electrolytes affect osmosis?

The relative

amounts of all these electrolytes ultimately controls osmosis -the flow of water between the fluid compartments. Too much sodium, for example, will drive water into the extracellular compartment and increase its volume. The increase in volume of the intravascular compartment, causes an increase in blood pressure. The opposite effect

occurs when the concentrations of potassium and magnesium increase.

What will a rise in intracellular electrolytes lead to?

A rise of intracellular electrolytes will drive fluid out of the extracellular compartment and into body cells.

What are basal metabolic rates?

Basal metabolic rates indicate the total amount of energy the body consumes at rest per unit of time. BMRs can be further broken down and calculated per organ.

Which organ is the most metabolically expensive to maintain, relative to its size?

The kidney!

What factors affect the metabolic costs for osmoregulation increase?

The first, and foremost important, is the difference between the osmolarity of the animal's body fluids and its environment. The second factor is how easily water or solutes cross in and out of the animal's body surface. The more permeable the skin, the harder it is to maintain a fixed gradient. The third factor is the amount of ATP required to operate the active transport pumps involved with moving solutes in or out of the animal's body.

How do the osmolarity of an animal's internal fluids and that of the environment affect energy use?

The closer the osmolarity of the animal's internal fluids to its environment, the less energy is required to maintain the gradient.

Do freshwater animals osmoregulate? Why or why not?

All freshwater animals are osmoregulators; the solute concentration of freshwater is simply too low to enable the proper functioning of neurons and muscle cells. Freshwater animals must therefore osmoregulate to maintain an electrolyte composition much higher than that of their environment.

Are terrestrial animals osmoregulators? Why or why not?

All terrestrial animals are osmoregulators. Ingesting foods and fluids disrupts the balance of internal electrolytes, eliminating the option for osmoconforming in terrestrial animals.

Do most invertebrates osmoregulate? Why or why not?

The high and stable osmolarity of most marine environments gives opportunity for most invertebrates and some vertebrates to osmoconform.

How would an ectothermic osmoconforming animal's metabolic rate compare to that of an endothermic osmoregulating one?

An ectothermic osmoconforming animal would therefore have a substantially lower metabolic rate relative to an endothermic osmoregulating animal of equal size.

Why are fish gills important, in addition to functioning in gas exchange?

In addition to functioning in gas exchange, fish gills also maintain electrolyte homeostasis and eliminate nitrogenous wastes.

How do fish achieve such diversity in their gills?

To achieve this diversity in function, the surface area of the gill epithelia represents 50% of the animal's total surface area.

What is a primary challenge of gills of marine fish?

For marine fish, the main challenge is losing water by osmosis through the gills and other body surfaces. As blood courses through the blood capillaries in the gill filaments, water quickly escapes towards the hyperosmotic environment.

How do marine fish compensate for water loss?

To compensate, marine fish drink lots of seawater and produce little urine.

What is the main issue with marine fish consuming so much salty water? How do active transport pumps play a role in this?

The downside of drinking so much salty water is the burden it placed on the epithelial cells of the gill lamella to pump salt out against its concentration gradient into the surrounding sea water. To support such a function requires the metabolically expensive operation of active transport pumps.

How are chloride pumps important for gill function?

Chloride pumps embedded in the epithelial cells of the gill lamella push chloride ions out by active transport and sodium ions follow passively. Other electrolytes that accumulate in body fluids are eliminated by the excretory system. These electrolytes include magnesium and calcium.

What is the body's principal metabolic rate?

Nitrogenous wastes are the body's principal metabolic waste.