Anatomy & Physiology · Chapter 21

The Lymphatic & Immune System

Every day, trillions of bacteria, viruses, and fungi test your borders. Most days you never notice. A layered defense force turns them away before they can take hold. This chapter follows that force, from its pipes to its sharpest weapons. First you will meet the lymphatic system. It is a quiet network that drains your tissues, carries fat from your meals, and houses your immune army. Then you will watch immunity, your body's power to fight off germs, unfold in three lines of defense. It starts with the skin on the outside. Next comes the fast fury of swelling and heat. Last come the slow, exact, lasting powers of T cells and B cells. By the end, you will know how you fight infection. You will also see why a vaccine protects you for years, but a shot of snakebite antivenom does not last.

By the end of this chapter you can

Describe the three functions of the lymphatic system. Trace the path of lymph from a tissue back to the blood.
Tell apart the primary and secondary lymphoid organs, and say what each one does.
Compare innate and adaptive immunity. Place each of the body's defenses in the right line of defense.
Explain the inflammatory response. Name its four main signs and link each sign to its cause.
Compare the jobs of helper and cytotoxic T cells. Explain why T cells need antigen presentation.
Describe how a B cell becomes a plasma cell. Describe the shape of an antibody and how antibodies stop a germ.
Tell a primary immune response from a secondary one. Explain why memory cells make vaccines work.
Sort immunity as active or passive and natural or artificial. Give a real example of each of the four types.

Section 1The lymphatic system

**Reference.** The organs and vessels of the lymphatic system — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A drainage network with three jobs

Your blood vessels are not perfectly sealed. Blood pushes through the tiniest vessels, the capillaries. As it does, a little fluid squeezes out into the spaces between cells. About three liters a day more fluid leaks out than flows back in. If that fluid just pooled, you would swell up. Your blood volume would crash within hours.

The lymphatic system is the return line that stops this. It is a one-way network of vessels. It collects the leaked fluid, called interstitial fluid, which means "fluid between cells." Once that fluid is inside a lymphatic vessel, it gets a new name: lymph. The system hands the lymph back to the blood near the heart. That is its first and most basic job: fluid balance.

It also does two more jobs. The small intestine is lined with special lymph capillaries called lacteals. They soak up fats and fat-soluble vitamins from your food. These are too large to enter the blood capillaries directly. The milky, fatty lymph they carry is called chyle.

The third job matters most for this chapter. The lymphatic system is the home and highway of the immune system. It houses the lymphocytes and macrophages, the white blood cells that fight infection. It also filters lymph through checkpoints. There, those cells can catch invaders in the act.

Function	What happens	Where
Fluid balance	Returns about 3 L a day of leaked fluid to the blood	Lymph capillaries all over the body
Fat absorption	Takes up fats and fat-soluble vitamins from food as chyle	Lacteals in the small intestine
Immune defense	Houses and moves lymphocytes; filters lymph for germs	Lymph nodes, spleen, and other lymphoid organs

How lymph flows — with no pump

Lymph starts out in lymphatic capillaries. These are tiny tubes closed at one end. Their wall cells overlap like tiles and act as small one-way flaps. Fluid can push the flaps open to get in, but it cannot push its way back out. Next, lymph flows into larger lymphatic vessels. These look and act a lot like veins. They have thin walls and inner valves that stop backflow.

Along the way, the vessels pass lymph through bean-shaped lymph nodes. Then the vessels join into large lymphatic trunks. Last, they drain into two collecting ducts. The small right lymphatic duct drains the right arm and the right side of the head and chest. The large thoracic duct drains everything else. Both empty into the subclavian veins at the base of the neck. There, the fluid becomes part of the blood again.

Here is the catch. The lymphatic system has no heart of its own. It moves lymph the same way veins move blood back uphill. The skeletal muscle pump helps, as working muscles squeeze the vessels. The respiratory pump helps too, through pressure changes with each breath. Smooth muscle in the vessel walls also squeezes in a steady rhythm. The valves keep all of this flow going one way. This is why gentle movement lowers swelling. It is also why a limb that cannot move tends to puff up.

Blocked drainage means swelling. Sometimes lymph vessels are damaged or removed. For example, lymph nodes in the armpit may be taken out during breast cancer surgery. Then fluid can no longer drain, and the arm swells with protein-rich fluid. This is lymphedema. The same idea explains why lymph nodes get swollen and sore when you are sick. This swelling, called lymphadenopathy, is a classic sign that your immune system is fighting an infection nearby.

The lymphoid organs

The organs of the immune system fall into two groups, based on what they do. Primary lymphoid organs are where lymphocytes are born and grow up. One is the red bone marrow. All blood cells start there, and B lymphocytes mature there. The other is the thymus, an organ with two lobes in the chest. Here T lymphocytes finish maturing. They also learn not to attack the body's own tissues. The thymus is largest in childhood. It slowly shrinks after puberty.

Secondary lymphoid organs are where grown-up lymphocytes meet antigens and launch attacks. An antigen is any marker the immune system sees as foreign. The lymph nodes filter lymph. The spleen filters blood. Its red pulp removes worn-out red blood cells, and its white pulp checks the blood for germs. This group also includes the tonsils and the Peyer's patches in the intestine. Last is MALT (mucosa-associated lymphoid tissue). It is spread-out tissue that guards every wet surface of the body.

Interactive Tap each organ to meet it. The lymphatic system drains fluid, takes in fats, and houses the cells that fight infection. Open full ↑

Key points

The lymphatic system has three jobs. They are fluid balance (it returns leaked fluid), fat absorption (lacteals make chyle), and immune defense.
Lymph flows one way: capillaries → vessels → nodes → ducts → subclavian veins. Muscles and breathing move it, not a pump. Valves keep it going one way.
Primary lymphoid organs (bone marrow, thymus) make lymphocytes. Secondary organs (lymph nodes, spleen, tonsils, MALT) are where they fight.

Section 2The three lines of defense

The three lines of immune defense — **Reference.** The layered lines of immune defense — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Two kinds of immunity, three layers of defense

Immunity comes in two broad styles. Innate (nonspecific) immunity is the defense you are born with. It treats every intruder the same way. It responds within minutes to hours. It keeps no memory of past attackers.

Adaptive (specific) immunity is the defense you build over your life. It aims at one certain germ with great precision. It takes days to get going the first time. Most important, it remembers. These two styles are set up as three lines of defense. Each line takes over if the one before it is broken.

The first line is the body's outer wall. It is made of the skin, the mucous membranes, and the fluids they give off. Healthy skin is a dry, tough layer that few germs can cross. Mucous membranes trap invaders in sticky mucus. Then tiny hairs called cilia sweep them out.

Chemicals back up the wall. Sweat and skin oil form a mildly sour coating called the acid mantle. Tears and saliva hold lysozyme, an enzyme that breaks down bacterial walls. Harsh stomach acid kills many germs you swallow. The normal flora, your harmless resident bacteria, crowd out dangerous ones. Nothing here aims at one germ. It is a barrier against all of them.

When the wall is breached

Sometimes a germ gets past the barrier. It may come in through a cut, a burn, or a droplet you breathe in. Then the second line meets it inside the body. This line is still innate and still nonspecific. But now the fight uses cells and chemicals.

Phagocytes are cells that swallow and digest invaders. Neutrophils and macrophages are both phagocytes. Natural killer (NK) cells destroy virus-infected cells and tumor cells on sight. The second line also includes the inflammatory response and fever. It uses germ-fighting proteins too. Interferons warn nearby cells about viruses. Complement is a chain of blood proteins. It punches holes in germs and tags them to be destroyed. The second line does not care which germ it is fighting. It only cares that something does not belong.

The third line is adaptive immunity. It is made of the lymphocytes, which are the T cells and B cells. It also includes the antibodies that B cells make. This line is slow to start, but it is extremely exact. It also leaves behind memory cells. So the next time the same germ shows up, the attack is faster and stronger. Most of this chapter is about the second and third lines.

Line	Type	Defenses
First line	Innate: outer barriers	Skin, mucous membranes, and their fluids (acid, lysozyme, mucus, normal flora)
Second line	Innate: inside the body	Phagocytes, NK cells, inflammation, fever, interferons, complement
Third line	Adaptive: specific	T lymphocytes, B lymphocytes, antibodies, memory cells

Innate buys time; adaptive wins the war. The first two lines act right away. They hold the invader back for the several days the third line needs. In that time, the third line builds a custom weapon. Only the third line has memory. That is why you catch measles only once. It is also why a vaccine can protect you for decades.

Interactive Tap each line. You will see a wall that keeps invaders out, fast all-purpose troops, and a slow but exact special force. Open full ↑

Key points

Innate immunity is fast and general, and it has no memory. Adaptive immunity is slow at first and specific, and it remembers.
The first line is outer barriers. The second line is inner innate defenses (phagocytes, NK cells, inflammation, fever, complement). The third line is lymphocytes and antibodies.
Only the third line has memory. This is the basis of lifelong immunity and of vaccines.

Section 3The inflammatory response

The steps of the inflammatory response — **Reference.** Steps of the inflammatory response — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Four signs you already know

Cut your finger, and within minutes it turns red, warm, swollen, and sore. These are the four cardinal signs, or main signs. In Latin they are rubor, calor, tumor, and dolor. They are not the infection itself. They are your body's inflammatory response, a key weapon of the second line of defense.

Inflammation starts whenever tissue is hurt or invaded. It may feel like something is going wrong. In fact, it is a carefully planned rescue. It has three goals. It works to get rid of debris and germs. It tries to wall off the hurt area so infection cannot spread. And it sets the stage for repair.

What actually happens

As soon as tissue is hurt, damaged cells send out chemical alarm signals. So do nearby mast cells, which are immune cells that live in the tissue. The best-known signal is histamine. Others include prostaglandins. These chemicals do two things at once.

First, they cause vasodilation, which means the blood vessels widen. More blood rushes into the area. That causes the redness and the heat. Second, they make the capillary walls leakier. Fluid, clotting proteins, and defense cells seep out into the tissue. That causes the swelling, also called edema. The pressure of the swelling presses on nerve endings. So do the chemicals themselves. Together they cause the pain.

Now the cleanup crew arrives. Chemical signals from the injury draw phagocytes toward it. This is called chemotaxis. Neutrophils arrive first. They squeeze out of the blood through the vessel wall, a step called diapedesis. Then they swallow germs. Longer-lived macrophages come next. They eat germs and dead cells.

Sometimes a thick, creamy pus collects. It is just the leftover mess of the battle. It holds dead neutrophils, broken tissue, and destroyed germs. Once the threat is gone, the same spot becomes a workshop where repair begins.

Cardinal sign	Latin	What causes it
Redness	Rubor	Vasodilation brings more blood to the area
Heat	Calor	More blood flow warms the tissue
Swelling	Tumor	Leakier capillaries let fluid seep into the tissue
Pain	Dolor	Swelling pressure plus alarm chemicals acting on nerve endings

Inflammation can also spread to the whole body. This is called systemic inflammation. When infection is widespread, chemicals called pyrogens reset the brain's thermostat, the hypothalamus, to a higher setting. This causes fever. A moderate fever protects you. It slows the growth of many germs. It also speeds up the body's repair work. The bone marrow makes more white blood cells too. This is called leukocytosis. That is why a high white-cell count on a blood test hints at infection.

Fever is a feature, not just a bug. Within limits, a mild fever helps you fight infection. So it is not always wise to bring every fever down. Also note how anti-inflammatory drugs work. NSAIDs and aspirin block prostaglandins. That is exactly why they ease redness, swelling, pain, and fever all at once.

Interactive Step through inflammation. See why a cut turns red, warm, swollen, and sore. Open full ↑

Key points

The four cardinal signs are redness, heat, swelling, and pain. They come from widened blood vessels (vasodilation) and leakier capillaries.
Histamine from mast cells starts it. Then neutrophils and then macrophages arrive by chemotaxis and diapedesis. They swallow debris and germs.
Inflammation gets rid of threats, walls off the area, and prepares for repair. Across the whole body, it can cause helpful fever and leukocytosis.

Section 4T cells & cell-mediated immunity

Types of T lymphocytes in cell-mediated immunity — **Reference.** T lymphocyte types in cell-mediated immunity — Harlé, G.; Kowalski, C.; Garnier, L.; Hugues, S., CC BY 4.0 (via Wikimedia Commons)

Immunity that acts cell-to-cell

The third line splits into two branches. One branch is cell-mediated immunity, run by T lymphocytes. It deals with threats that hide inside your cells. These include viruses, some bacteria, and cancer cells. T cells work by direct cell-to-cell contact. They do not use antibodies.

T cells are born in the bone marrow. But they finish growing up in the thymus. (The "T" stands for thymus.) In the thymus, they are tested. Only cells that leave your own tissues alone are allowed to graduate.

Here is the key rule of T cells. They are blind to free-floating antigen. A T cell can only see an antigen when it is presented to it. That means another cell holds it up on its surface. It sits in a special display molecule called an MHC (major histocompatibility complex).

Every cell with a nucleus carries MHC class I. On it, the cell shows samples of the proteins it is making inside. A healthy cell shows its own normal proteins. But a virus-infected cell shows bits of the virus. In effect, it raises a flag that says "I am infected."

Some cells are antigen-presenting cells (APCs). These are dendritic cells, macrophages, and B cells. They also carry MHC class II. On it, they show bits of germs they have swallowed. This need for presentation is a safety switch. It keeps the powerful T-cell response from firing at random.

The T-cell lineup

Each T cell has a receptor that fits one antigen. Sometimes a T cell finds the presented antigen that matches it. That one cell is then chosen to multiply. It makes an army of identical copies, called clones. This process is called clonal selection. The clones grow into several special types:

T cell type	Marker	Job
Helper T cell	CD4	The coordinator. It reads antigen on MHC II and releases cytokines, or signal chemicals. These switch on cytotoxic T cells, B cells, and macrophages.
Cytotoxic T cell	CD8	The killer. It reads antigen on MHC I and destroys infected or cancer cells with perforin and granzymes.
Regulatory T cell	CD4	The brake. It calms the response and shuts it down, which prevents autoimmunity.
Memory T cell	—	The archive. It lasts for years and mounts a fast response if the germ comes back.

The helper T cell is the quarterback of the whole adaptive response. It sends signals called cytokines. Without them, neither cytotoxic T cells nor B cells fully switch on.

The cytotoxic T cell is the assassin. It finds an infected cell and releases perforin, which punches a hole in the target. Then it sends in granzymes. These make the cell destroy itself, a process called apoptosis. The germ dies along with its hiding place.

Why HIV is so devastating. The human immunodeficiency virus (HIV) infects and destroys helper T (CD4) cells. Take out the quarterback, and the whole adaptive team falls apart. Both the cytotoxic T cells and antibody making collapse. That is why doctors track the CD4 count. It shows how close a person is to AIDS.

Interactive Tap each type. T cells run cell-mediated immunity. They coordinate and kill only when an antigen is presented to them. Open full ↑

Key points

T cells mature in the thymus. They run cell-mediated immunity by direct contact, not with antibodies.
T cells act only when antigen is presented on an MHC molecule. MHC I shows what is inside any cell. MHC II is used by APCs.
Helper (CD4) cells coordinate the whole response. Cytotoxic (CD8) cells kill infected cells. Regulatory cells put on the brakes. Memory cells remember.

Section 5B cells & antibodies

B cells and the shape of an antibody — **Reference.** B cells and the structure of an antibody — סתו כסלו, CC BY-SA 4.0 (via Wikimedia Commons)

The antibody factories

The other branch of adaptive immunity is humoral immunity. The name comes from humor, an old word for body fluid. It is the job of B lymphocytes. T cells attack threats hidden inside cells. B cells fight germs floating free in the blood, lymph, and tissue fluid. They do not fight hand to hand. Instead, they make antibodies.

B cells mature in the red bone marrow. Here is a handy memory trick: B is for bone marrow. Then they wait in the lymph nodes and spleen for their matching antigen.

A B cell has a receptor on its surface. Sometimes that receptor locks onto its own antigen. Usually a helper T cell also gives it a push with cytokines. Then the B cell switches on. It goes through clonal selection and divides fast into two kinds of cells.

Most become plasma cells. These are short-lived but amazing antibody factories. Each one can pump out thousands of identical antibodies every second. The rest become memory B cells. They stay around for years. They are ready to respond at once if the same germ ever comes back.

What an antibody is and how it fights

An antibody is a Y-shaped protein. It is also called an immunoglobulin, or Ig. It is built from four chains, two heavy and two light. The two tips of the Y form the variable region. This is a custom-shaped antigen-binding site. It fits one antigen like a key fits a lock. The stem is the constant region. It sets the antibody's class.

Antibodies do not usually kill a germ outright. Instead, they tag it and disable it. They work in four ways. They neutralize toxins and viruses by covering the part that would stick to your cells. They agglutinate, or clump, many germs together so they are easy to clear away. They activate complement, which bursts the invader. And they opsonize. This means they coat the target so phagocytes can grab it more easily.

Class	Where / when	Notable job
IgG	The most common kind; main antibody of the secondary response	Long-term immunity; the only class that crosses the placenta
IgM	The first antibody made in a new infection	Large, made of five units (a pentamer); strongly switches on complement
IgA	Body fluids like saliva, tears, mucus, and breast milk	Guards mucous membranes and passes immunity to a baby in milk
IgE	Attached to mast cells and basophils	Drives allergies and fights parasitic worms
IgD	On the B-cell surface	Acts as an antigen receptor that helps switch on the B cell

Why the second exposure is different

The first time you meet a germ, you get a primary response. It is slow. It takes several days to find the right B cell. That cell must then multiply and start making antibody. At first it is mostly IgM, and then IgG. You may get sick in the meantime.

But this first meeting leaves behind memory B cells. If the same germ comes back, you get a secondary response. It is much faster and stronger. Memory cells release a flood of high-quality IgG within a day or two. They often clear the invader before you ever feel sick. That difference is the whole idea behind vaccination. A vaccine safely builds the memory now. Then the real infection meets a secondary response.

Memory is why one exposure protects. A newborn is shielded for months. IgG crosses the placenta, and IgA arrives in breast milk. The mother lends her antibodies to her baby. But that borrowed protection fades, because the baby made no memory cells of its own. Only an active response, from illness or a vaccine, builds lasting memory.

Interactive Step through how one B cell becomes an antibody factory in the humoral response. Open full ↑

Key points

B cells mature in bone marrow and run humoral immunity. They become plasma cells that make huge numbers of antibodies, and memory B cells that remember.
An antibody is a Y-shaped protein with a variable region that binds antigen. It disables germs by neutralization, agglutination, complement activation, and opsonization.
The secondary response is fast and strong because of memory cells. This is the idea behind every vaccine.

Section 6The four types of immunity

Active and passive, natural and artificial immunity — **Reference.** The four categories of acquired immunity — SGUL lymres, CC BY-SA 4.0 (via Wikimedia Commons)

Two questions sort every kind of immunity

Acquired immunity is the kind you gain during life. You can sort it by asking two simple questions. The first question is: who made the antibodies?

If your own immune system made them, the immunity is active. It is slow to build, but it lasts a long time, because it comes with memory cells. If you got ready-made antibodies from someone else, the immunity is passive. It protects you right away, but only for a while. Those borrowed antibodies break down, and there are no memory cells to replace them.

The second question is: how did the exposure happen? If it came through everyday life, it is natural. If it came through medicine, it is artificial. Combine the two questions, and you get four types.

	Active (you make it)	Passive (you receive it)
Natural	You catch an infection, get well, and make your own antibodies and memory	Antibodies pass from mother → child across the placenta (IgG) or in breast milk (IgA)
Artificial	A vaccine shows you a harmless form of the antigen, so you build antibodies and memory	A shot of ready-made antibodies (antiserum), such as antivenom or rabies or tetanus immunoglobulin

When each one is used

Active immunity is the goal of prevention. You can earn it the hard way, by surviving an illness. That is naturally acquired active immunity. Or you can earn it the safe way, through a vaccine. That is artificially acquired active immunity. Either way, the payoff is the same. You gain a lasting supply of memory cells. They are ready to crush the germ if you ever meet it again. This is why childhood vaccines can protect for decades.

Passive immunity is the tool for emergencies. A fetus cannot wait to build its own defenses. So it borrows its mother's antibodies. That is naturally acquired passive immunity. Now think of a person just bitten by a venomous snake or a rabid animal. They cannot wait the weeks a vaccine would take. So they get a shot of ready-made antibodies for instant protection. That is artificially acquired passive immunity. The trade-off is always the same. Passive immunity works right now, but it fades in weeks to months. It never leaves memory behind.

Active prevents; passive rescues. Say you have time and want lasting protection. Then you want active immunity, from a vaccine. Now say you have already been exposed and need protection this minute. Then you want passive immunity, from an antibody shot. One builds memory slowly. The other lends protection instantly, but only for a short time.

Interactive Tap a box. Immunity is sorted by who makes the antibodies (active vs passive) and how you were exposed (natural vs artificial). Open full ↑

Key points

In active immunity, your body makes the antibodies. It is slow and lasting, and it has memory. In passive immunity, you get antibodies from another source. It is instant and short-lived, with no memory.
Natural exposure happens through life. Artificial exposure happens through medicine. Together, these give four types in all.
Vaccines give artificially acquired active immunity. Antivenom and antibodies from mother to baby are passive.

The chapter in one breath

The lymphatic system returns leaked fluid to the blood, takes in fat from food as chyle, and houses immune cells. It moves lymph one way with no pump, using valves and muscles.
Primary lymphoid organs (bone marrow, thymus) build lymphocytes. Secondary organs (lymph nodes, spleen, tonsils, MALT) are where they fight.
Defense comes in three lines: outer barriers, then inner innate defenses, then specific adaptive immunity. Only the last line has memory.
Inflammation (redness, heat, swelling, pain) is the second line at work. Widened vessels and leaky capillaries bring in phagocytes. They clear threats and start repair.
T cells run cell-mediated immunity. They act only on presented antigen. Helper cells coordinate, and cytotoxic cells kill infected cells.
B cells run humoral immunity. They become plasma cells that make antibodies, and memory cells that make the secondary response fast. This is the basis of vaccines.
Immunity is active or passive and natural or artificial. Active immunity is slow and lasting, with memory. Passive immunity is instant and short-lived, without memory.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Active immunity: Long-lasting immunity where your own body makes the antibodies and memory cells. You gain it naturally through infection or artificially through a vaccine.
Antibody (immunoglobulin): A Y-shaped protein made by plasma cells. It binds one antigen and tags it to be destroyed by neutralization, agglutination, complement, or opsonization.
Antigen: Any molecule the immune system sees as foreign and can respond to. It is usually a piece of a germ.
Antigen-presenting cell (APC): A cell that shows swallowed antigen on MHC class II to switch on helper T cells. Dendritic cells, macrophages, and B cells are APCs.
B lymphocyte (B cell): A lymphocyte that matures in bone marrow and runs humoral immunity. It turns into antibody-making plasma cells and memory cells.
Complement: A chain of blood proteins in the innate second line. It bursts germs, boosts inflammation, and coats germs so phagocytes eat them.
Cytotoxic T cell (CD8): A T lymphocyte that sees antigen on MHC I. It kills infected or cancer cells using perforin and granzymes.
Helper T cell (CD4): A T lymphocyte that sees antigen on MHC II. It coordinates the adaptive response by releasing cytokines, and it is the main target of HIV.
Humoral immunity: Adaptive immunity carried out by antibodies in body fluids and run by B cells. It works against germs floating outside cells.
Inflammation: An innate response to injury or infection, marked by redness, heat, swelling, and pain. It clears threats and prepares tissue for repair.
Innate immunity: The general defenses you are born with, which are the first and second lines. They respond fast to any threat and keep no memory.
Lymph: Fluid from between the cells once it has entered a lymph vessel. It is on its way back to the blood.
Lymph node: A small, bean-shaped secondary lymphoid organ. It filters lymph and houses lymphocytes and macrophages.
Lymphocyte: A white blood cell of the lymphoid line. T cells and B cells carry out adaptive immunity, and natural killer (NK) cells act in innate immunity.
Macrophage: A large, long-lived phagocyte that swallows germs and debris. It can also present antigen to T cells.
Memory cell: A long-lived T or B cell left after an infection. It allows a fast, strong secondary response if the germ comes back.
MHC (major histocompatibility complex): Surface molecules that show antigen to T cells. MHC I is on all cells with a nucleus, and MHC II is on antigen-presenting cells.
Passive immunity: Short-term immunity from ready-made antibodies you get from another source (placenta, breast milk, or a shot). It works at once but leaves no memory.
Plasma cell: A switched-on B cell that makes and releases huge numbers of antibodies, thousands per second.
Thymus: A primary lymphoid organ in the chest where T lymphocytes mature. There they are tested so they will not attack the body's own tissues.
Vaccine: A harmless form of an antigen that triggers artificially acquired active immunity. It builds antibodies and memory without causing disease.

Check yourself

1. Name the three jobs of the lymphatic system. Which one keeps you from swelling up over the course of a day?

Fluid balance, fat absorption, and immune defense. Fluid balance prevents swelling. About 3 L of fluid leaks out of the blood capillaries each day. The lymphatic system returns it to the blood. That keeps tissues from getting waterlogged.

2. The lymphatic system has no pump of its own. So how does lymph move? What keeps it flowing only one way?

The skeletal muscle pump, the respiratory pump, and smooth muscle squeezes move it. One-way valves keep it going in one direction. Like veins, lymph vessels rely on movement around them, not a central pump. Inner valves stop backflow.

3. Sort these into primary and secondary lymphoid organs: thymus, spleen, red bone marrow, lymph nodes. What defines each group?

Primary: thymus and red bone marrow (where lymphocytes form and mature). Secondary: spleen and lymph nodes (where lymphocytes meet antigen and respond). B cells mature in bone marrow and T cells in the thymus. The spleen filters blood, and lymph nodes filter lymph.

4. What is the single biggest difference between innate and adaptive immunity? Which line of defense provides each?

Adaptive immunity is specific and has memory. Innate immunity is general and has none. The first and second lines are innate. They include barriers, then phagocytes, inflammation, fever, and complement. The third line (T cells, B cells, antibodies) is adaptive.

5. You breathe in a germ, and it slips past the mucous membranes. Name three second-line defenses it now faces.

Any three of these: phagocytes (neutrophils or macrophages), natural killer cells, inflammation, fever, interferons, complement. These inner innate defenses act fast and treat all germs alike once the outer barrier is broken. They buy time for the adaptive third line.

6. A cut turns red, warm, swollen, and painful. Link each of these four cardinal signs to the blood vessel change that causes it.

Redness and heat come from vasodilation (more blood flow). Swelling comes from leakier capillaries (fluid seeps out). Pain comes from that swelling plus alarm chemicals pressing on nerve endings. Histamine from mast cells causes both the widening and the leakiness.

7. Why can a moderate fever actually help you? And how does an NSAID like aspirin reduce inflammation?

A moderate fever slows germ growth and speeds tissue repair. NSAIDs work by blocking prostaglandins. Pyrogens raise the hypothalamus's temperature setting to cause fever. Prostaglandins drive pain and swelling. So blocking them eases several signs at once.

8. Why can a T cell never respond to a free-floating antigen on its own? What molecule solves this?

A T cell only sees antigen when it is presented on an MHC molecule. MHC I on any cell with a nucleus shows what is being made inside it. MHC II on antigen-presenting cells shows swallowed material. This rule keeps T cells from firing at random.

9. Compare the jobs of a helper T cell and a cytotoxic T cell. Why is losing helper T cells (as in HIV) so harmful?

Helper (CD4) cells coordinate the response with cytokines. Cytotoxic (CD8) cells kill infected cells directly with perforin and granzymes. Helper cells switch on both cytotoxic T cells and B cells. So destroying them makes the whole adaptive response collapse. That is the core of AIDS.

10. What does a switched-on B cell turn into? Describe the parts of the antibody it makes.

It divides into plasma cells (antibody factories) and memory B cells. The antibody is a Y-shaped protein. It has a variable antigen-binding region at the tips and a constant region that forms the stem. The variable region gives the antibody its lock-and-key fit for one antigen.

11. Why is the secondary immune response so much faster than the primary one? How do vaccines use this?

Memory cells from the first exposure release a fast, strong flood of antibodies the next time. A vaccine safely makes those memory cells ahead of time. Then the real germ meets a fast secondary response, not a slow primary one.

12. Sort each one as active or passive, and natural or artificial: (a) a childhood measles vaccine; (b) antibodies passed to a fetus across the placenta; (c) an antivenom shot after a snakebite; (d) getting over chickenpox.

(a) artificial active; (b) natural passive; (c) artificial passive; (d) natural active. The active types (a and d) build your own long-lasting memory. The passive types (b and c) lend ready-made antibodies. They protect right away, but only for a while.

Previous← Blood Vessels & Circulation NextThe Respiratory System →