Biol 222 Exam 1 Study Guide
Biol 222 Exam 1 Study Guide BIOL 222
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This 15 page Study Guide was uploaded by Amanda Tobias on Sunday September 18, 2016. The Study Guide belongs to BIOL 222 at Towson University taught by Dr. Colleen Winters in Fall 2016. Since its upload, it has received 63 views. For similar materials see Human Anatomy and Physiology II in Biology at Towson University.
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Date Created: 09/18/16
A&P II Lecture I Homeostasis (Chapter 1) Fall 2016 Vocabulary: Equilibrium: a dynamic state in which two opposing forces or processes are in balance Effector: system, organ, gland that carries out action to correct the problem Extrinsic: activities of a cell are adjusted by nervous system or endocrine system. They are external systems that help correct a problem Receptor: Monitor for changes in a specific parameter Homeostasis: Maintenance of a stable internal environment that is required for survival Autoregulation: cells, tissues, organs, or organ systems automatically adjust their activities in response to a change in their enviornment Integration: processing of sensory information what needs to be done to solve the problem Intrinsic: activities of a cell are adjusted locally without the help of the endocrine or nervous system Dynamic Equilibrium: a state of balance between two continuous processes. The process must stay within a certain range Regulatory mechanisms maintain homeostasis; in order to do this the system: Must be able to sense the parameter and detect changes (Receptors) Must be able to process the information, relate the information to other available information and “make a decision” about what the appropriate response should be (Integration Center) Must be able to act, send signal, to produce an effect (Effector) Example: Thermostat in your house or apartment (Fig 1.2) Example: Homeostatic compensation for postural change (Figure) Negative Feedback – When the effector reverses or opposes the stimulus it is called negative feedback. (Fig 1.3) Positive feedback is much less common and results in an amplification of the situation; (Fig 1.4) How is homeostatic regulation accomplished? o Autoregulation (intrinsic) o Extrinsic regulation Review questions: 1. Define homeostasis. Maintenance of a stale internal environment that is required for survival 2. Which mechanism of homeostatic regulation always involves the nervous or endocrine system? Extrinsic 3. Why is homeostatic regulation important to an organism? The loss of homeostatic regulation leads to tissue/organ damage and death. The stable environment that homeostasis maintains is required for survival. 4. Explain the function of negative feedback systems. In negative feedback the effector eliminates or opposes the stimulus in order to fix the homeostatic problem. For example if body temperature goes up, we sweat to cool it down. Negative feedback helps to maintain homeostasis. 5. What happens to the body when homeostasis breaks down? When homeostasis breaks down, body systems stop working how they should, and that leads to tissue damage and organ failure. Eventually, if homeostasis breaks down it leads to death. 6. Explain how a positive feedback system works. Positive feedback results in a rapid change in the same direction as the stimulus. The initial stimulus produces a response that amplifies the stimulus. This continuous until the stimulus is eventually removed. Examples are child birth, blood clotting, and fever. The response is in the same direction rather than the opposing direction 7. Why is positive feedback helpful in blood clotting but unsuitable for the regulation of body temperature? In order to regulate body temperature, if body temperature goes up, the stimulus most create a response for it to go down. That is negative feedback. However, in blood clotting in order to fix the problem, clotting occurs and the chemical that leads to blood clotting signals for more of the chemical to be made. It is in the same direction and is amplified, unlike in temperature regulation. 8. What is a dynamic equilibrium? 9. Which of the following is not an example of negative feedback? a. increased pressure in the aorta triggers mechanisms to lower blood pressure b. a rise in blood calcium levels triggers the release of a hormone that lowers blood calcium levels c. a rise in estrogen during the menstrual cycle increases the number of progesterone receptors in the uterus d. increased blood sugar stimulates the release of a hormone from the pancreas that stimulates the liver to store blood sugar A&P 214 Endocrine Lecture Fall 2016 Vocabulary Peptide: a compound consisting of amino acids in a chain Hormone: compounds that alter cellular processes by changing the types, activities, or amounts of proteins/enzymes present or being made by the target cells Transcription: the process of converting DNA into RNA Chronic: persisting for a long time or consistently reoccurring Protein: macromolecule consisting of one or more long chains of amino acids folded together Hydrophilic: water loving Translation: the process of converting RNA to protein Acute: sharp, having a short and relatively severe course Paracrine: a chemical or hormone that only affects or travels to nearby cells Hydrophobic: repels water Amplification: increase in strength Aggravate: to make worse or more serious Endocrine: chemicals that are released into the blood stream and travel to the area that needs them. Specificity: the selective attachment or influence of one substance on another, for example receptors specificity for the hormone that binds to them Modulation: the capacity to regulate Dysfunction: abnormality or impairment in the function of a specific organ or system Exocrine: relating to glands that secrete their products through an opening in the epithelium rather than into the bloodstream Saturation: when no more of something can be combined, absorbed, or added Stress: any situation that disrupts homeostasis or threatens out physical well being Mechanisms of celltocell communication (Table 18.1) • Direct communication: communication between cells that are touching each other • Synaptic communication: communication between neurons with action potentials, short term but fast response • Paracrine communication: chemicals travel locally to nearby cells • Endocrine communication – hormones: chemicals released into the blood stream bind to specific receptors Overview of the endocrine system (Fig 18.1) What do hormones do? Hormones alter cellular processes by changing gthe types, activities, or amounts of proteins/enYmes present or being made by the target cells Hormone structure (figure) • Steroids (Figure): derived from cholesterol, hydrophobic • Monoamines: amino acid derivatives that are synthesized by tyrosine or tryptophan o Catecholamines: hydrophilic o thyroid hormones: hydrophobic o pineal hormone:hydrophobic • Peptide hormones o Synthesized from DNA translation o Small – 8 to 20 amino acids long o Glycoproteins Large chains with carbohydrates hydrophilic Hormone transport Hydrophilic hormones can mix in blood but cannot cross cell membranes Hydrophobic hormones bind to hydrophilic transport proteins to travel through blood, but they can cross cell membranes Hormone receptors o Steroid and Thyroid Hormone receptors [Fig 18.4a, b] o Peptides and Catecholamines (membrane hormone receptors) Second messengers Adenylate cyclase [Fig 18.3a] o Hormone binds to membrane receptor o Hormone receptor complex activates G protein o Active G protein activates adenylate cyclase o Adenylate cyclase converts ATP to cAMP o cAMP initiates a cascade of enzyme activations that lead to the cell response Calcium [Fig 18.3b] o calmodulin o hormone binds to the membrane receptor o hormone receptor complex activates G protein o G protein activates PLC, PLC breaks membrane phospholipid into IP3 and DAG o Calcium binds to calmodulin o Calcium/calmodulin activates enzyme cascade that leads to cell response o Amplification [Figure] Modulation of target cell sensitivity • Upregulation: increased receptors • Downregulation: decreased receptors Hormone clearance Most hormones are degraded by the liver and kidney Hormones bound to transport proteins stay in blood for several weeks Free hormones last only a few minutes to hours Review questions 1. Define hormone. Compounds that alter cellular processes by changing the types, activities, or amounts of proteins/enzymes present or being made by the target cells 2. Describe paracrine communication. A cell produces a chemical that travels locally to nearby cells where a response to the chemical fixes the problem. For example, if your pinky needs more circulation, a chemical will be released that tells blood vessels to expand in order to give it more oxygen. 3. Identify four mechanisms of intercellular communication. Direct communication (gap junctions), synaptic communication (action potentials), paracrine communication, endocrine communication 4. How could you distinguish between a neural response and an endocrine response on the basis of response time and duration? A neuron response is fast but short term, and an endocrine response is slow but it lasts a long time 5. How would the presence of a substance that inhibits the enzyme adenylate cyclase affect the activity of a hormone that produces its cellular effects by way of the second messenger cAMP? The hormone would not be able to produce a response. Adenylate cyclase converts ATP to cAMP, which initiates the cascade of enzyme activity that eventually leads to cell response. If the adenylate cyclase is inhibited, the rest of the activity can’t take place. 6. What primary factor determines each cell’s hormonal sensitivities? The type and amount of receptors on the cells surface 7. What is the primary difference in the way the nervous and endocrine systems communicate with their target cells? The nervous system sends an action potential so the response is fast, but the action potential is gone as soon as it passes on to the next neuron. The endocrine system releases a hormone that travels through the blood to the target cell, where it binds to the receptor on the surface or the DNA in the nucleus. Then there is a series of events that lead to the cell response. The response time is slower, but the hormone stays in the body for longer than the action potential in the nervous system. 8. In what ways can a hormone modify the activities of its target cells? It can activate or deactivate genes in the nucleus or mitochondria, or it can use secondary messengers such as cAMP, cGMP, or Calcium to trigger a cascade of enzymes. 9. A researcher observes that stimulation by a particular hormone induces a marked increase in the activity of G proteins in the target plasma membrane. The hormone being studied is probably a. a steroid c. testosterone e. aldosterone b. a peptide d. estrogen 10. Why do corticosteroids and thyroid hormones require transport proteins to travel in the bloodstream? Steroids and thyroid hormones are hydrophobic, and blood is mostly made of water, so the transport proteins are required to protect them in the presence of water in the blood. 11. Explain how one hormone molecule can activate millions of enzyme molecules. When a hormone binds to a receptor, that receptor can activate all the G proteins near it, which can activate all the adenylate cyclases near by each G protein if it’s the cAMP second messenger, and each adenylate cyclase can convert an ATP to a cAMP, and each cAMP can initiate a cascade of enzymes that leads to the cell response. The response is amplified. FLUID COMPARTMENTS and ELECTROLYTE BALANCE Fall 2016 Vocabulary Extracellular: between cells Hypotonic: more water than ions Distension: enlarged, swollen from internal pressure Intracellular: Within in the cell Hypertonic: In comparing two solutions, it is the solution with the higher osmolarity Hemorrhage: blood loss, to bleed Osmosis: The movement of water across a selectively permeable membrane from one solution to another solution that contains a higher solute concentration Osmolarity: the total concentration of dissolved materials in a solution, regardless of their specific identities, expressed in moles Semipermeable: only certain molecules or ions can pass through the membrane Ion: an atom or molecule having a positive or negative charge due to the loss or gain, respectively of one or more electrons, sometimes called electrolyte Total body water (TBW) (Fig 27.1b) Fluid Compartments Intracellular Fluid (ICF): Fluid inside the cell, 60% of total body water Extracellular Fluid (ECF): Fluid outside of the cells o Interstitial Fluid: fluid between cells, 25% of ECF o Blood Plasma and Lymph: 8% o Small percentage of water found in other places: CSF, synorial fluid, vitreous and aqueous humor, endolymph and perilymph, 7% Composition of the fluids in each compartment varies but water is continually exchanged by OSMOSIS between the compartments, by way of capillary walls and cell (plasma) membranes [Figure]] The ECF and ICF compartments are in osmotic equilibrium (due to movement of water) and it is important to consider the components that cause osmotic pressure (electrolytes and colloids). The concentration of solutes is the same but composition is different o Additions and deletions from the ECF of water and salt are immediately compensated for by water movement to or from the ICF. Fluid Shift [fig 27.3, 4) Fluid Balance Regulation of Intake: osmoreceptors respond to increased osmolarity of ECF and the presence of angiotensin II. A 23% osmolatiry increase stimulates thirst. Regulation of Output Antidiuretic hormone (ADH) or vasopressin: released when osmotic concentration of ECF goes up. Stimulates water conservation in kidneysreabsorb more water Aldosterone: released if potassium goes up and sodium goes down in the ECF. Stimulates sodium conservation in kidneys Disorders of water balance [Table] Fluid deficiency o Two kinds Volume depletion (hypovolemia): both water and sodium decrease (osmolarity is the same but volume is decreased). Caused by hemorrhage, severe burns, chronic vomiting, diarrhea Dehydration: water decreases but sodium is normal (osmolarity goes up and volume goes down)hypertonic. Caused by insufficient fluid intake, diabetes, overuse of diuretics Fluid excess o Less common o Two types Volume excess – H O2and Na are retained, ECF isotonic but high volume Hypotonic hydration (water intoxication) – more H O is retained or ingested than + 2 Na ; ECF becomes hypotonic o Natriuretic peptides (ANP and BNP): released when ECF volume goes up too high, stimulates loss of water in kidneys ELECTROLYTE BALANCE o Important because o Total electrolyte concentrations affect water balance between fluid compartments + o Concentrations of individual ions can affect cell function and system function (Na nervous system; Ca and K cardiac and skeletal muscle) o Sodium balance [Fig 27.6] o Hypernatremia; >145 mEq/L o Example 2: Hyponatremia; < 130 mEq/L o Potassium balance (Figure) o Hyperkalemia – too much K+ in the ECF > 5.5 mEq/L; o Hypokalemia – too low K+ in the ECF < 3.5 mEq/L; o Calcium balance o Hypercalcemia (>5.8mEq/L) o Hypocalcemia (<4.5 mEq/L) Review questions: 1. Identify the fluid compartments in the body. Give examples of what components make up each. Intracellular fluid is made up of the fluid inside the cells. Extracellular fluid is made up of interstitial fluid, blood plasma, and other fluids. 2. Some tumors of the brain, pancreas, and small intestine secrete ADH (vasopressin). What type of water imbalance would this produce and why? ADH stimulates water conservation in the kidneys, so this would produce a fluid excess. 3. Suppose there were no mechanisms to stop the sense of thirst until the blood became sufficiently hydrated. Explain why we would routinely suffer hypotonic hydration. It takes 30 minutes for the water to reach the ECF after you drink it, so if there were no mechanisms to stop the sense of thirst you would keep drinking water until a half hour after you started. Therefore, water levels would continue to increase without sodium levels changing. 4. Name and define the four types of fluid imbalance, and give an example of a situation that could produce each type. Volume depletion could be caused by chronic vomiting, dehydration could be caused by insufficient fluid intake, volume excess could be caused by renal failure, and hypotonic hydration could be caused drinking way too much water. 5. Name three hormones that play a major role in adjusting fluid and electrolyte balance in the body? ADH, Aldosterone, and Natriuretic peptides 6. What effect would being in the desert without water for a day have on the osmotic concentration of your blood plasma? The osmotic concentration would increase 7. Why does prolonged sweating increase plasma sodium ion levels? Sweating causes a greater loss of water than sodium ions, so prolonged sweating would cause a person to lose more water than sodium ions from their ECF. 8. Which are more dangerous, disturbances of sodium balance or disturbances of potassium balance? Explain your answer. Disturbances of potassium balance are more dangerous because imbalances are rare so the body isn’t used to dealing with them, unlike sodium imbalances. They can cause cardiac arrest and death among other problems. Chapter 19 – Blood A&P II Fall 2016 Vocabulary Plasma: The fluid fround substance of whole blood; what remains after the cells have been removed from a sample of whole blood Albumin: maintains osmotic pressure in blood Fibrinogen: clotting factor Hematocrit:percent of whole blood made up of red blood cells, white blood cells, and platelets Buffy coat: white blood cells and platelets Viscosity: the resistance to flow that a fluid exhibits as a result of molecular interaction within the fluid Hemopoiesis: blood cell formation and differentiation Erythropoiesis: formation of red blood cells Erythrocyte: red blood cell Erythropoietin:a hormone released by most tissues, especially by the kidneys, when oxygen levels decreases, stimulates red blood cell formation in red bone marrow Hemolysis: destruction of red blood cells Hypoxia: deficiency in the amount of oxygen reaching the tissues Antibody: a globular protein produced by plasma cells that will bind ot specific antigens and promote their destruction or removal from the boyd Coagulation: process by which blood clots Hemostasis: the stopping of the flow of blood Thrombus: a blood clot that forms in a vein Embolus: an object that has been carried through the blood to lodge in a vessel Functions of Blood Blood composition and Characteristics (Fig 19.1) What type of tissue is blood? o Plasma o Formed elements Red blood cells (RBC) or Erythrocytes White blood cells (WBC) or Leukocytes Platelets Plasma: Composition o Plasma proteins Viscosity and osmolarity – colloid osmotic pressure o Solutes o Water RED BLOOD CELLS (RBCS) (figure) Hematocrit o How is this determined? Blood is centrifuged to separate into red blood cells and plasma, and the percent of red blood cells is determined Hemoglobin o Structure (Fig 19.3) Hematopoiesis Hemopoietic stem cell: o Myeloid stem cells o Lymphoid stem cells Erythropoiesis (Fig 19.5) RBC death and disposal (Fig 19.4) Hemolysis Erythrocyte disorders Blood types – Determined by the presence or absence of specific surface proteins RBCs (act as antigens) (Fig 19.6a) Type A Type B Type AB Type O Yes No No Yes Type A Type B No Yes No Yes Type AB Yes Yes Yes Yes Type O No No No Yes Type O universal donor; Type ABnuniversal receiver Our blood type is also given with a + or – o Rh+ for the presence of D surface antigens on RBCs o Rh for the absence of D surface antigens on RBCs o (Fig 19.8) WHITE BLOOD CELLS (WBCs) or LEUKOCYTES (Fig 19.9) Characteristics Types o Granulocytes Neutrophils Eosinophils Basophils o Agranulocytes Monocytes Lymphocytes (B cells, T cells, NK cells) Fig 19.10 Abnormalities Hemostasis o Platelets (thrombocytes) Phases of hemostasis o Vascular spasm (Fig 19.11a) o Platelet plug formation (Fig 19.11b) platelet aggregation platelet activation o Coagulation phase (Fig 19.11c) Goal of clotting is for thrombin to convert the fibrinogen into fibrin Clinical note Thrombosis embolus Fibrinolysis Review questions: 1. List the major functions of blood. Transport of dissolved gases, nutrients, waste, and hormones, regulation of pH and ion composition, restrict fluid loss at injury sites, transport of immune system cells, and stabilize body temperature 2. Identify the composition of formed elements in blood. Erythrocytes, Leukocytes, and platelets 3. What two components make up whole blood? Plasma and water 4. List the three major types of plasma proteins. Albumins, globulins, and fibrinogen 5. What would be the effects of a decrease in the amount of plasma proteins? The hematocrit would go up 6. Describe hemoglobin. Molecule in red blood cells that carries oxygen 7. How would the hematocrit change after an individual suffered a significant blood loss? The hematocrit would stay the same but the volume of blood in the body would decrease 8. What is the function of surface antigens on RBCs? They attack antibodies of foreign blood types 9. Which blood type can be safely transfused into a person with type O blood? O blood 10. Why can’t a person with type A blood safely receive blood from a person with type B blood? The person with type A blood has antibodies that attack the surface protein of B blood 11. What is the primary function of platelets? Platelets help with blood clotting 12. What is hemopoiesis? Blood cell formation and differentiation 13. What is the role of erythropoietin in the regulation of RBC production? It is the hormone released when oxygen levels decrease that stimulates red blood cell formation in the red bone marrow 14. What happens to each component of an RBC and its hemoglobin when it dies and disintegrates? Fragments are digested by macrophages in the liver and spleen and become iron, feces, and free amino acids 15. Describe the cause, prevention and treatment of hemolytic disease of the newborn. This occurs when the fetus is Rh+ and the mother is Rh. Something causes fetal blood cells to enter the mother’s bloodstream during delivery. The mother produces antibodies to the fetal Rh+ cells. If her next pregnancy is with an Rh+ fetus, antibodies to D antigen cross the placenta and destroy fetal RBCs. The fetus could get anemia or die. RhoGam is used to destroy antigens in the mother so this doesn’t happen. 16. In what respect does blood clotting represent a negative feedback loop? What part of it is a positive feedback loop? The platelet plug formation is a positive feedback loop, and it is a negative feedback loop because the increased blood loss leads to a response that decreases blood loss 17. Identify the five types of white blood cells. Neutrophils, basophils, eosinophils, lymphocytes, and monocytes 18. What is the overall function of leukocytes? Overall function is immune response for the immune system. Chapter 22 IMMUNITY Fall 2016 Vocabulary Antigen: a substance capable of inducing the production of antibodies Barrier: a primary line of defense which physically impedes the entry of foreign particles Phagocytosis: The engulfing of extracellular materials or pathogens: the movement of materials or pathogens: the movement of extracellular materials into the cytoplasm by enclosure in a membranous vesicle Immunity: resistance to infection and disease caused by foreign substances, toxins, or pathogens Epitope: The part of an antigen molecule to which an antibody attaches itself Antibody: a globular protein produced by plasma cells that will bind to specific antigens and promote their destruction or removal from the body Mobilize: to make something capable of movement Costimulation: the delivery of a second signal from an antigen presenting cell to a T cell, which rescues an activated T cell form anergy, allowing it to produce lymphokines necessary for production of additional T cells Perforate: pierced with holes Virus: a noncellular pathogen Variable: able to change or be adapted Constant: not able to change or be adapted Surveillance: patrolling an area Nonspecific Defenses and Specific Defenses NonSpecific defenses (Fig 22.11) Physical barriers Phagocytes Natural Killer Cells (immune surveillance) Perforins [Fig 22.12] Interferons Complement [old figures] Inflammation – Fig 22.15 Fever Specific Defens (Fig 22.16) Characteristics of the specific defenses o Specificity o Memory o Coordinated by Tcells (cell mediated) and Bcells (antibody mediated) Agranulocytes Lymphocytes o T (thymusderived) cells o B (bonederived) cells o Active acquired immunity: induced in the host itself by antigen and lasts a long time, sometimes lifelong o Passive acquired immunity: acquired through transfer of antibodies or activated Tcells from an immune host and is short lives, usually lasting only a few months o CELLMEDIATED IMMUNITY: coordinated by Tcells; Types of cells Cytotoxic T cells Helper T cells Regulatory T cells Memory T cells How does it work? Antigen presentation o Major Histocompatibility Complex (MHC) proteins Class I MHC (Fig 22.18a): an infected cell will display both normal and abnormal proteins on its membrane with MHC Class I proteins Class II MHC (Fig 22.18c): a macrophage phagocytizes an antigen and displays fragments on the surface with Class II MHC proteins Antigen Recognition (Fig 22.19) Activation and Attack o (Fig 22.19, old figure)cytotoxic T cells respond only to MHC Class I o (Fig 22.20) helper T cells respond only to MHC Class II HUMORAL (ANTIBODY)MEDIATED IMMUNITY : coordinated by Bcells o Antibodies (old figure) Sensitization of the B cell (Fig 22.22, old figure) Clinical Note: Immune System Disorders (Fig 22.1, old figure) Hypersensitivity Autoimmune diseases Immundeficiency Review questions: 1. List the body’s innate (nonspecific) defenses. Skin, mucous epithelia, attributes of selected areas(mouth, lungs, stomach, small intestine, urethra, female genitalia 2. What types of cells would be affected by a decrease in the number of monocyteforming cells in red bone marrow? This would result in fewer macrophages of all types 3. A rise in the level of interferon in the body suggests what kind of infection? Viral infection 4. Explain the difference between cellmediated (cellular) immunity and antibodymediated (humoral) immunity. In cellmediated immunity, T cells defend against abnormal cells and pathogens inside cells. In antibodymediated immunity, B cells secrete antibodies that defend against antigens and pathogens in body fluids 5. Identify the two forms of active immunity and the two forms of passive immunity. The two forms of active immunity are naturally acquired active immunity and artificially induced active immunity. The two forms of passive immunity are naturally acquired passive immunity and artificially induced passive immunity 6. Identify the major types of T cells. Cytotoxic T cells, memory T cells, Helper T cells, and Suppressor T cells 7. A decrease in the number of cytotoxic T cells would affect which type of immunity? Cell mediated immunity 8. How would a lack of helper T cells affect the antibodymediated immune response? Without helper T cells, the antibody mediated immune response would probably not occur, because helper t cells promote B cell division, the maturation of plasma cells, and antibody production. 9. Describe the structure of an antibody. An antibody molecule is made of two parallel pairs of polypeptide chains, one pair of heavy chains and one pair of light chains. Each chain contains both constant segments and variable segments 10. A sample of lymph contains an elevated number of plasma cells. Would you expect the number of antibodies in the blood to be increasing or decreasing? Why? You would expect the number of antibodies in the blood to be increasing because plasma cells produce and secrete antibodies 11. What are the signs of inflammation and what causes each? The signs of inflammation are redness, swelling, heat and pain. They can be caused by impact, abrasion, distortion, chemical irritation, infection by pathogens, and extreme temperatures. 12. What role does an antigenpresenting cell play in the activation of a T cell? An antigenpresenting cell stimulates T cells to become either cytotoxic cells or helper cells 13. How is a cytotoxic T cell like a natural killer cell? How are they different? Cytotoxic T cells seek out and destroy abnormal and infected cells. They are highly mobile cells that roam throughout injured tissues. When a cytotoxic T cell encounters its target antigen, it immediately destroys the target cell. Natural killer cells also kill their target cells, but they do so without the stimulation of antigens 14. What is the difference between a B cell and a plasma cell? B cells can connect to antigens right on the surface of the invading virus or bacteria and plasma cells can only connect to virus antigens on the outside of the cell.
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