Immunity, Inflammation & Hypersensitivity
The immune system is the body’s standing army — mostly invisible, until it wins too slowly, overreacts, or turns on its own side. This chapter follows the whole arc: who the invaders are, how the innate and adaptive defenses answer them, what inflammation costs, and what happens when immunity misfires as allergy, autoimmunity, or immunodeficiency.
By the end of this chapter you can
- Name the major classes of pathogens and how each causes disease.
- Contrast innate with adaptive immunity, and humoral with cell-mediated defense.
- Match each of the five antibody classes to its signature role.
- Explain the cardinal signs of inflammation by their mechanisms.
- Tell the four hypersensitivity types apart with a classic example of each.
- Describe how autoimmunity and immunodeficiency (including HIV/AIDS) arise.
Section 1The invaders: types of pathogens
Right now, as you read this, you are covered and colonized by trillions of microorganisms, and that is completely normal. We call these resident microbes your normal flora (or microbiota). They live on your skin, in your mouth, and especially in your gut, and they earn their keep: they crowd out troublemakers, help digest food, and even make some vitamins (like vitamin K). Most of the time you and your flora have a peaceful, mutually helpful relationship. A pathogen is different. It is any organism that can cause disease, either by invading tissue, releasing toxins, or triggering an immune response that harms you. Some organisms are opportunistic: harmless in their usual spot, but dangerous when they get somewhere they do not belong (for example, gut bacteria entering the bloodstream after surgery). Understanding what kind of invader you are dealing with is the first step, because the type of organism decides the treatment.
Bacteria: tiny, but they play dirty
Bacteria are single-celled organisms with their own machinery to live and reproduce, which is why many can grow on their own outside our cells. Nurses sort them first by shape (round cocci, rod-shaped bacilli) and, crucially, by the Gram stain. In plain terms, gram-positive bacteria have a thick peptidoglycan cell wall (and no outer membrane) that traps and holds the purple stain, while gram-negative bacteria have a thin wall hidden under an extra outer membrane, so they lose the purple and take a pink counterstain. That difference is not just cosmetic. Gram-negative bacteria carry endotoxin (lipopolysaccharide, LPS) in that outer membrane; when the bacteria die and break apart, LPS floods out and can drive high fever, dangerously low blood pressure, and septic shock. Bacteria harm us in two broad ways: by invasion (physically spreading into and destroying tissue) and by releasing toxins. Exotoxins are proteins some bacteria secrete that damage cells at a distance (think of the toxins behind tetanus or C. difficile colitis), while endotoxin comes from the gram-negative wall itself.
Viruses: hijackers that cannot live alone
Viruses are not really cells at all. They are genetic material (DNA or RNA) wrapped in a protein coat, and they are obligate intracellular parasites meaning they must get inside one of your living cells to do anything. A virus attaches to a host cell, enters it and releases its genes inside, and then hijacks the cell's own ribosomes and enzymes to mass-produce new virus copies. The cell is turned into a factory and is often destroyed in the process. Because viruses hide inside your cells and borrow your machinery, antibiotics do nothing against them, and treatment is harder to target without harming the host.
Fungi, parasites, and a word on prions
Fungi (yeasts like Candida and molds) are more complex cells that often turn pathogenic when the immune system is weakened or when normal flora is wiped out by antibiotics. Protozoa are single-celled parasites (for example, the Plasmodium that causes malaria), while helminths are the multicellular parasitic worms. Finally, prions are the strange outliers: misfolded proteins, with no DNA or RNA at all, that cause other proteins to misfold, leading to fatal brain diseases.
Matching the treatment to the invader
Here is the payoff: naming the class of organism guides the whole plan. Antibiotics target features only bacteria have (like the cell wall), so they are useless against viruses. Giving them anyway breeds antibiotic resistance and can trigger a C. difficile infection by wiping out protective flora.
| Pathogen | Key trait | Treatment class |
|---|---|---|
| Bacteria | Own cells; toxins & invasion | Antibiotics |
| Viruses | Hijack host cells | Antivirals |
| Fungi | Yeasts & molds | Antifungals |
| Parasites | Protozoa & worms | Antiparasitics |
Explore the major classes of pathogens in the figure below to see how these families line up side by side.
- Normal flora usually protect us; a pathogen is any organism that causes disease, and opportunists turn harmful in the wrong place or when defenses drop.
- Bacteria harm us by invasion and toxins; gram-negative endotoxin (LPS) can drive fever and septic shock, while the Gram stain guides early therapy.
- Viruses are obligate intracellular parasites that hijack host cells to replicate, so antibiotics do not work on them.
- Fungi, protozoa, and helminths each need their own drug class; prions are misfolded proteins with no nucleic acid.
- Matching drug to organism (antibiotic vs antiviral vs antifungal) matters, and misusing antibiotics breeds resistance and can cause C. difficile.
Section 2The body’s defenses
Every second of every day, your body is fending off bacteria, viruses, and other invaders without you ever noticing. It does this with two teams that work together: a fast, general-purpose crew called innate immunity, and a slower but far more precise crew called adaptive immunity. Understanding how these two teams operate is the foundation for everything you will learn about infection, inflammation, and hypersensitivity.
Innate immunity: fast, but nonspecific
Innate immunity is the defense you are born with. It responds within minutes to hours, but it does not care exactly who the invader is: it treats most threats the same way. Think of it as the building's locks, alarms, and security guards.
- Physical and chemical barriers come first. Intact skin is a wall invaders cannot easily cross, and mucous membranes lining the airways and gut trap microbes so cilia and secretions can sweep them out. Stomach acid and lysozyme in tears add chemical defense. This is why a break in the skin, like an IV site or a pressure injury, is a real infection risk.
- Phagocytes are cells that eat invaders. Neutrophils arrive first and fast (they dominate the early response and are the main cell in pus), while macrophages arrive a bit later, engulf debris, and clean up. This eating process is called phagocytosis.
- Complement is a cascade of plasma proteins that punches holes in microbes, tags them for phagocytes (opsonization), and amplifies inflammation.
- Natural killer (NK) cells destroy your own cells that have turned dangerous, especially virus-infected and some tumor cells.
- Fever is a whole-body innate response. Chemical messengers called pyrogens reset the hypothalamus to a higher temperature, which slows microbial growth and speeds up immune cells. This is why a fever, though uncomfortable, is often a sign the defenses are working.
Adaptive immunity: specific, with memory
Adaptive immunity is slower to start (days on a first exposure) but it is specific: it targets one particular invader, and it remembers. It has two branches.
- Humoral immunity is run by B cells. When activated, B cells become plasma cells that pump out antibodies (immunoglobulins) into the blood and fluids. Antibodies neutralize toxins, clump microbes together, and flag them for destruction. This branch is especially good against invaders outside your cells, like bacteria in the bloodstream.
- Cell-mediated immunity is run by T cells and handles threats inside cells, such as viruses, and abnormal cells.
The T-cell coordinators and killers
Two T-cell types are worth knowing by name. Helper T cells (CD4) are the coordinators: they release cytokines that activate B cells, cytotoxic T cells, and macrophages. Because HIV attacks CD4 cells, a falling CD4 count leaves the whole system leaderless, which is why HIV is so devastating. Cytotoxic T cells (CD8) are the killers: they directly destroy virus-infected and cancerous cells.
Active vs passive immunity
Immunity can be earned or borrowed. In active immunity your own body mounts the response and builds memory, either by getting sick or by vaccination (a safe preview of the invader). It is slow to develop but long-lasting. In passive immunity you receive ready-made antibodies from someone else, so protection is immediate but temporary, because you never made memory cells. Examples include antibodies crossing the placenta to the fetus and antibodies delivered in breast milk.
| Feature | Active immunity | Passive immunity |
|---|---|---|
| Source | Your own immune response | Borrowed antibodies |
| Onset | Slow (days to weeks) | Immediate |
| Duration | Long-lasting (memory) | Short (no memory) |
| Examples | Infection, vaccines | Placenta, breast milk, antibody injections |
Primary vs secondary response
The primary response is your body's first meeting with an invader. It is slow, often taking a week or more to build up antibodies, which is why you can feel sick during that first encounter. But the immune system files away memory cells. On a repeat exposure, the secondary (memory) response is faster and stronger, producing far more antibodies far more quickly, so you may never feel sick at all. This is exactly why vaccines and booster doses work: they safely create memory so the real invader triggers a rapid secondary response. Explore the innate-versus-adaptive figure below to see how these teams fit together.
- Innate immunity (barriers, neutrophils and macrophages, complement, NK cells, fever) is fast but nonspecific; adaptive immunity is slower but specific with memory.
- Adaptive immunity has two branches: humoral (B cells make antibodies, best against extracellular threats) and cell-mediated (T cells, best against intracellular threats).
- Helper T cells (CD4) coordinate the response; cytotoxic T cells (CD8) kill infected and abnormal cells.
- Active immunity (infection or vaccine) is self-made and long-lasting; passive immunity (placenta, breast milk, antibody injections) is borrowed, immediate, and temporary.
- The secondary (memory) response is faster and stronger than the primary response, which is the basis for vaccines and boosters.
Section 3Antibodies (immunoglobulins)
Your immune system faces a huge problem: how do you recognize millions of different germs, when each one looks a little different? The answer is antibodies, also called immunoglobulins (Ig). Think of an antibody as a tiny Y-shaped protein tag that your body custom-builds to match one specific target. The tips of the Y are the variable region, shaped to lock onto a particular antigen (a molecular fingerprint on a microbe). The stem of the Y, the constant region, is the handle that the rest of the immune system grabs.
Antibodies are made by plasma cells, which are mature B lymphocytes that have been activated by their antigen. Once released, an antibody does three main jobs. First, it tags the invader so that phagocytes recognize and eat it, a process called opsonization. Second, it can neutralize a threat by physically coating a virus or a toxin so it can no longer attach to your cells. Third, it can activate complement, a cascade of blood proteins that punches holes in bacterial membranes and amplifies inflammation. In short: tag, neutralize, and activate.
The five classes and their signature jobs
All antibodies share that Y shape, but the constant-region handle comes in five flavors, and each flavor sends the antibody to a different job and a different place in the body.
- IgG is the most abundant antibody in blood and the workhorse of the secondary (memory) response. It is the only class that crosses the placenta, and it is excellent at opsonization, neutralization, and activating complement.
- IgM is the first to appear in a new infection and the largest antibody, because it circulates as a pentamer (five Y units joined together). Those ten binding arms make it a powerful early complement activator.
- IgA guards the mucosal surfaces, the wet linings of the gut, lungs, and airways, and it is concentrated in secretions like saliva, tears, and breast milk, giving a nursing newborn ready-made protection.
- IgE drives allergy and type I hypersensitivity and defends against parasites. It binds mast cells and basophils; when antigen cross-links it, those cells release histamine.
- IgD mostly sits on the surface of naive B cells as part of the B-cell receptor, helping them get activated.
Reading antibody levels: recent versus past infection
Because IgM and IgG appear on different timelines, a lab panel can tell a story about when an exposure happened. When you meet an antigen for the first time, IgM rises first, usually within days, then fades over a few weeks. IgG rises later and stays elevated for months to years, providing lasting immunity. Nurses use this pattern constantly.
| Feature | IgM elevated | IgG elevated |
|---|---|---|
| What it suggests | Recent or current infection | Past infection or immunity |
| Timing | Appears first, fades in weeks | Appears later, persists long-term |
| Response type | Primary response | Secondary / memory response |
| Classic example | Acute hepatitis or rubella | Prior vaccination or old exposure |
So IgM positive with IgG negative usually points to a fresh infection, while IgG positive with IgM negative usually means the person was infected or vaccinated in the past and is now immune.
Why placenta-crossing IgG protects a newborn
A newborn's own immune system is still learning and cannot make strong antibody responses yet. Because IgG crosses the placenta during pregnancy, the mother essentially loans her baby a full library of antibodies. This passive immunity shields the infant for roughly the first several months of life while their own B cells mature. IgA in breast milk adds a second layer of protection right at the gut lining. The trade-off is that this borrowed protection is temporary, which is exactly why the infant vaccination schedule begins before it fully wears off.
The interactive figure below lets you explore the five classes side by side.
- Antibodies (immunoglobulins) are Y-shaped proteins from plasma cells that tag, neutralize, and activate complement against a specific antigen.
- Five classes, memory hook GAMED: IgG (most abundant, crosses placenta), IgA (mucosa and breast milk), IgM (first and largest, pentamer), IgE (allergy and parasites), IgD (B-cell receptor).
- IgM rises first and fades, signaling a recent infection; IgG rises later and persists, signaling past infection or immunity.
- Maternal IgG crosses the placenta to give the newborn temporary passive immunity for the first months of life.
Section 4Inflammation & healing
Think of inflammation as your body's first-responder team. It is not the same as infection. Infection means microbes have invaded; inflammation is the response, and it fires whether the trigger is bacteria, a splinter, a burn, a sprain, or dying tissue. In healthy tissue, capillaries run quietly and white blood cells patrol the bloodstream. The moment cells are injured, that calm is interrupted on purpose: the goal is to contain the threat, clear the debris, and set up repair. Understanding the response means understanding a handful of chemical mediators and what they do to blood vessels.
The five cardinal signs and why each happens
Injured cells and nearby mast cells release histamine, along with prostaglandins, bradykinin, and cytokines. These mediators drive two vascular changes that explain every classic sign.
- Redness (rubor) and heat (calor) come from vasodilation - the arterioles widen, so more warm blood floods the area.
- Swelling (tumor) comes from increased capillary permeability. The vessel walls loosen and let protein-rich fluid leak into the tissue, forming edema.
- Pain (dolor) comes from bradykinin and prostaglandins stimulating nerve endings, plus pressure from the swelling.
- Loss of function (functio laesa) is the sum of pain and swelling - a swollen, aching joint simply will not move well.
Meanwhile, neutrophils are the first white cells to arrive, squeezing out of vessels (diapedesis) to begin phagocytosis of pathogens and debris.
Acute versus chronic inflammation
Acute inflammation is fast and self-limiting - hours to a few days - and ideally ends in full healing. Chronic inflammation lingers for weeks, months, or years, usually because the trigger never clears (persistent infection, autoimmune disease, ongoing irritation). The cell cast changes too: neutrophils give way to macrophages and lymphocytes. The cost to the tissue is real - chronic inflammation drives fibrosis (scarring), tissue destruction, and sometimes granuloma formation, so the organ slowly loses working cells and gains stiff, non-functional scar.
Exudates and the systemic response
The fluid that leaks out is called exudate. Serous exudate is thin and watery (think of a blister); purulent exudate is thick pus, full of dead neutrophils and microbes, and signals infection. Inflammation is not only local. Cytokines such as IL-1 and IL-6 travel to the brain and liver, producing systemic signs: fever (a raised hypothalamic set-point), leukocytosis (elevated WBC count), and rising acute-phase reactants - a climbing CRP (a true acute-phase protein), and a rising ESR (an indirect marker driven by fibrinogen), are lab clues that inflammation is active somewhere in the body.
Healing: resolution versus repair
Two outcomes are possible. Resolution is the best case: the tissue was only mildly injured and the cells can regenerate, so it returns to normal. Repair with scar (fibrosis) happens when damage is extensive or the cells cannot regenerate - fibroblasts lay down collagen and granulation tissue fills the gap, leaving a scar that holds but does not fully function.
| Feature | Primary intention | Secondary intention |
|---|---|---|
| Wound edges | Clean, approximated (sutured) | Wide, gaping, cannot be closed |
| Tissue loss | Minimal | Significant |
| Healing time | Fast | Slower |
| Scar | Small | Larger, more granulation tissue |
| Infection risk | Lower | Higher |
Why anti-inflammatory drugs work
NSAIDs (ibuprofen, aspirin) block the COX enzymes that make prostaglandins, so they blunt pain, fever, and swelling but leave much of the immune response intact. Corticosteroids (prednisone) act much higher upstream, suppressing the genes for many mediators at once - they are far more powerful but also more immunosuppressive, which is why they raise infection risk and slow healing. Explore the acute inflammatory response and its cardinal signs in the figure below.
- The five cardinal signs (redness, heat, swelling, pain, loss of function) all trace back to vasodilation and increased capillary permeability driven by histamine and prostaglandins.
- Acute inflammation is short and usually heals; chronic inflammation persists and costs tissue through fibrosis and destruction.
- Systemic signs of inflammation include fever, leukocytosis, and elevated CRP/ESR; purulent exudate suggests infection.
- Healing is either resolution (back to normal) or repair with scar; clean wounds heal by primary intention, gaping wounds by secondary.
- NSAIDs block prostaglandins (COX); corticosteroids suppress mediators broadly and can mask signs of infection.
Section 5When immunity overreacts: hypersensitivity
Your immune system is built to protect you, but sometimes it protects too hard. Hypersensitivity means the immune response is exaggerated or misdirected, so the defense itself becomes the source of tissue damage. To understand this, remember how normal immunity works. Your body makes antibodies (immunoglobulins) that tag foreign invaders, and specialized T cells that recognize threats directly. Normally these tools attack germs and leave your own cells alone. In hypersensitivity, that precision breaks down. The immune system reacts to something harmless (pollen, a food, a medication), targets your own cells, or leaves inflammatory debris behind that damages healthy tissue.
Clinicians sort these overreactions into four classic types, and a simple memory device keeps them straight: ACID - Anaphylactic, Cytotoxic, Immune-complex, Delayed. The first three are driven by antibodies; only the fourth is not.
Type I - Immediate (IgE-mediated): allergy and anaphylaxis
This is the classic allergy. On first exposure, the body makes IgE antibodies against an otherwise harmless allergen and parks them on the surface of mast cells and basophils (this silent step is called sensitization). On the next exposure, the allergen bridges those IgE molecules, and the mast cells degranulate, dumping histamine and other mediators within minutes. Histamine causes vasodilation (redness, warmth, low blood pressure), increased capillary permeability (swelling, hives), and bronchoconstriction (wheezing). A local reaction is hay fever or hives; a whole-body reaction is anaphylaxis, where airway swelling and vasodilatory shock can kill quickly. This is why epinephrine is the first-line rescue - it reverses bronchoconstriction and vasodilation fast.
Type II - Cytotoxic (antibody against cells)
Here antibodies (usually IgG or IgM) bind directly to antigens on the surface of specific cells, marking them for destruction by complement and phagocytes. The classic example is a mismatched blood transfusion: give type A blood to a type B patient and the recipient's preformed antibodies attack the donor red cells, causing rapid hemolysis, fever, flank pain, and dark urine. Another example is hemolytic disease of the newborn, where an Rh-negative mother makes anti-Rh antibodies that cross the placenta and destroy an Rh-positive baby's red cells.
Type III - Immune-complex mediated
In Type III, antibody and antigen clump together into immune complexes that circulate and then deposit in tissues - joints, skin, kidney glomeruli, blood vessel walls. Once lodged, they activate complement and trigger inflammation right where they settle. This is the mechanism behind systemic lupus erythematosus (SLE) and serum sickness (a reaction to foreign proteins, such as certain antivenoms). The key difference from Type II: the antibody is not aimed at a fixed cell, it is floating debris that inflames wherever it lands. Timing is broad rather than fixed - a localized Arthus reaction appears within about 4 to 12 hours, classic serum sickness takes 7 to 14 days after the foreign protein is given (fever, rash, joint pain), and SLE is lifelong and relapsing. So a reaction showing up a week after antivenom or antithymocyte globulin is still Type III.
Type IV - Delayed (T-cell mediated)
Type IV is the odd one out - no antibody at all. Instead, sensitized T lymphocytes recognize the antigen and recruit macrophages, and because this cell-driven process takes time, the reaction peaks at 48 to 72 hours. Read a TB skin test (PPD) at 48-72 hours for exactly this reason - the firm raised induration is delayed T-cell inflammation. Other examples include contact dermatitis (poison ivy, nickel) and transplant (graft) rejection.
| Type | Mediator | Timing | Classic example |
|---|---|---|---|
| I - Anaphylactic | IgE, mast cells, histamine | Seconds to minutes | Anaphylaxis, hay fever |
| II - Cytotoxic | IgG/IgM against cell surface | Minutes to hours | Transfusion reaction, HDN |
| III - Immune-complex | Circulating antibody-antigen complexes | Hours to weeks | SLE, serum sickness |
| IV - Delayed | T cells (no antibody) | 48-72 hours | TB skin test, poison ivy |
Explore the four types side by side in the interactive figure below.
- ACID orders the four types: Anaphylactic (I), Cytotoxic (II), Immune-complex (III), Delayed (IV).
- Types I, II, and III are antibody-driven; Type IV is T-cell driven with no antibody.
- Type I needs prior sensitization - the severe reaction hits on re-exposure, and epinephrine is the anaphylaxis rescue.
- Type II targets specific cells (transfusion reaction, hemolytic disease of the newborn); Type III deposits floating complexes that inflame tissues (SLE, serum sickness).
- Type IV is delayed 48-72 hours - the reason a TB skin test is read days later.
Section 6Autoimmunity & immunodeficiency
Your immune system has an almost impossible job: it must attack anything foreign - bacteria, viruses, cancer cells - while leaving you completely alone. The ability to tell "self" from "non-self" is called self-tolerance, and it is built during immune development. As lymphocytes mature, any that react strongly against your own tissues are normally deleted or switched off (a process of central and peripheral tolerance). When that filtering breaks down, the immune system turns its weapons inward. This section covers what happens when tolerance fails (autoimmunity) and, on the flip side, when the immune system is too weak to defend at all (immunodeficiency).
When self-tolerance breaks down: autoimmunity
In autoimmune disease, the immune system loses self-tolerance and begins producing autoantibodies and self-reactive T cells that treat normal tissue like an invader. The result is chronic inflammation and tissue destruction at whatever the immune system targets. The trigger is usually a mix of genetic susceptibility (often certain HLA types) plus an environmental push such as an infection, hormones, or drugs. A helpful way to learn these diseases is to ask one question: what tissue is being attacked? Explore the interactive figure below to see how tolerance breaks down step by step.
| Disease | Target attacked | Classic result |
|---|---|---|
| SLE (lupus) | Many tissues (multi-system) - skin, joints, kidneys, blood | Butterfly rash, joint pain, lupus nephritis; ANA usually positive |
| Rheumatoid arthritis | Synovial joints (the joint lining) | Symmetric, warm, swollen joints; morning stiffness |
| Multiple sclerosis | Myelin around CNS nerves | Slowed nerve signals - weakness, vision changes, numbness |
| Type 1 diabetes | Beta cells of the pancreas | No insulin made - high blood glucose |
| Hashimoto thyroiditis | Thyroid (destroys it) | Hypothyroidism - fatigue, cold, weight gain |
| Graves disease | Thyroid (over-stimulates it) | Hyperthyroidism - heat intolerance, weight loss, exophthalmos |
Notice that Hashimoto and Graves hit the same organ with opposite outcomes. In Hashimoto, autoantibodies and T cells slowly destroy thyroid tissue, so hormone output falls. In Graves, the autoantibody actually mimics TSH and switches the gland on constantly, flooding the body with thyroid hormone. Same target, opposite mechanism - a favorite exam contrast.
The opposite problem: immunodeficiency
Sometimes the danger is not an overactive immune system but an underactive one. In immunodeficiency, part of the immune defense is missing or weakened, so ordinary germs become dangerous. There are two broad types:
- Primary (congenital) - the person is born with a genetic defect in immune development. These are usually rare and often show up in infancy with repeated, severe, or unusual infections.
- Acquired (secondary) - the immune system develops normally but is later damaged by something else: infection (HIV), chemotherapy, long-term steroids or other immunosuppressants, malnutrition, or aging. These are far more common.
HIV/AIDS and the CD4 count
The key example of acquired immunodeficiency is HIV (human immunodeficiency virus). HIV specifically infects and destroys CD4+ T-helper cells - the "quarterback" cells that coordinate nearly the entire immune response. As the virus replicates, the CD4 count steadily falls. A normal CD4 count is usually around 500 to 1,500 cells/mm³. When it drops below about 200 cells/mm³ (or an AIDS-defining opportunistic infection appears), the diagnosis becomes AIDS. At that point the body can no longer fend off opportunistic infections - organisms like Pneumocystis pneumonia that a healthy immune system would easily control. So the CD4 count is essentially a scoreboard of how much immune defense remains.
When inflammation goes body-wide: SIRS and sepsis
Inflammation is usually local and protective. But when an infection spills the inflammatory response into the whole bloodstream, the reaction becomes dysregulated and dangerous. SIRS (systemic inflammatory response syndrome) describes this body-wide inflammatory storm - often marked by abnormal temperature, high heart rate, fast breathing, and abnormal white-cell count. When infection triggers this body-wide response, it is called sepsis - and current definitions reserve that word for infection plus new organ dysfunction, not infection alone. Here the same mediators meant to fight germs cause widespread vasodilation and leaky vessels, dropping blood pressure and starving organs of oxygen - which can progress to septic shock and organ failure. It is a sobering reminder that too much immune activity can be as deadly as too little.
Why immunosuppressed patients are so fragile
Whether the cause is HIV, chemotherapy, transplant anti-rejection drugs, or steroids, an immunosuppressed patient has fewer or weaker defenders. Germs that would barely register in a healthy person can cause overwhelming infection, and the usual warning signs may be muted - a patient with neutropenia - an absolute neutrophil count below about 1,500/mm3, and severe below 500/mm3 - may not mount a fever or form pus, so infection hides until it is advanced. This is why nursing care emphasizes strict hand hygiene, neutropenic precautions, and treating even a low-grade fever as a possible emergency. Note the wording: it is the neutrophil count, not the total white-cell count, that sets the risk - a patient can carry a normal or even high WBC and still be dangerously neutropenic.
- Autoimmunity is a loss of self-tolerance - the immune system attacks the body's own tissue; learn each disease by asking what tissue is targeted (SLE = multi-system, RA = joints, MS = myelin, T1DM = beta cells, Hashimoto/Graves = thyroid).
- Hashimoto destroys the thyroid (hypothyroid) while Graves over-stimulates it (hyperthyroid) - same organ, opposite effect.
- Immunodeficiency is either primary (born with it) or acquired; HIV is the key acquired example, destroying CD4+ helper cells, and a CD4 count below ~200 cells/mm³ defines AIDS.
- SIRS/sepsis is a dangerous, body-wide inflammatory response to infection that can cause vasodilation, low blood pressure, and septic shock.
- Immunosuppressed patients catch infections easily and may show blunted signs (little fever or pus), so any fever is treated urgently.
The chapter in one breath
- Pathogens differ by structure and treatment: bacteria (cell wall, antibiotics), viruses (need a host cell, antivirals), fungi, protozoa/parasites, and misfolded-protein prions each demand a different defense and drug.
- Innate immunity is the fast, nonspecific first responder you are born with (skin, phagocytes, complement, fever) while adaptive immunity is slower but specific and remembers the invader through B cells (antibodies) and T cells.
- The five immunoglobulin classes split the work: IgG (most abundant, crosses the placenta), IgM (first responder, biggest), IgA (guards mucous membranes and breast milk), IgE (allergy and parasites), and IgD (B-cell surface receptor).
- Inflammation is the tissue-level alarm with five cardinal signs: redness (rubor), heat (calor), swelling (tumor), pain (dolor), and loss of function, all driven by vasodilation and increased capillary permeability from histamine and other mediators.
- The four hypersensitivity reactions run I (immediate, IgE-mediated, anaphylaxis), II (cytotoxic, antibody attacks cells, as in transfusion reactions), III (immune-complex deposition, as in lupus), and IV (delayed, T-cell-mediated, as in TB skin test and contact dermatitis).
- Autoimmunity is a loss of self-tolerance where the immune system attacks the body's own tissue, producing diseases like rheumatoid arthritis, systemic lupus erythematosus, and type 1 diabetes.
- Immunodeficiency is a weakened or missing defense that can be primary (congenital) or secondary (acquired from chemo, steroids, or malnutrition), leaving the patient prone to opportunistic infection.
- HIV is a retrovirus that destroys CD4+ helper T cells, crippling adaptive immunity; when CD4 counts fall below 200 cells/mm³ or opportunistic infections appear, the illness has progressed to AIDS.
ReferenceWords worth knowing
A quick reference for the terms in this chapter — skim it now, or circle back whenever a word trips you up.
- Anaphylaxis
- A severe, rapid, whole-body type I hypersensitivity reaction in which mass release of histamine causes widespread vasodilation, bronchoconstriction, and dangerously low blood pressure that can kill within minutes if untreated.
- Antibody
- A Y-shaped protein (also called an immunoglobulin) made by plasma cells that binds one specific antigen to tag it for destruction, neutralize it, or trigger other immune defenses.
- Antibody titer
- A lab measurement of how much of a specific antibody is in the blood, used to check immunity from vaccination or past infection or to track an active immune response.
- Antigen
- Any molecule, usually a protein or sugar on a germ or foreign cell, that the immune system recognizes as "not self" and responds to by making antibodies or activating T cells.
- Adaptive immunity
- The slower, learned arm of defense that targets a specific antigen and builds memory, so the next exposure to that same invader is met faster and harder.
- Autoimmunity
- A breakdown of self-tolerance in which the immune system mistakes the body's own healthy tissues for a threat and attacks them, as in lupus or rheumatoid arthritis.
- Cell-mediated immunity
- The arm of adaptive immunity driven by T lymphocytes rather than antibodies, which directly kills virus-infected or cancerous cells and coordinates other immune responses.
- Complement
- A cascade of blood proteins that, once triggered, punches holes in microbe membranes, coats germs to mark them for eating, and amplifies inflammation.
- Cytokine
- A small signaling protein (such as an interleukin or interferon) that immune cells release to communicate, calling in reinforcements and directing the strength and type of the response.
- Exudate
- The protein-rich fluid, cells, and debris that leak from blood vessels into inflamed tissue, ranging from watery (serous) to pus-filled (purulent) depending on the cause.
- Humoral immunity
- The arm of adaptive immunity carried out by B lymphocytes and the antibodies they secrete, which circulate in body fluids to neutralize pathogens and toxins.
- Immunodeficiency
- A weakened or missing part of the immune system, whether inherited or acquired (as in HIV or chemotherapy), that leaves a person prone to frequent or unusual infections.
- Inflammation
- The body's protective response to injury or infection, producing the classic redness, heat, swelling, and pain as blood vessels widen and immune cells rush to the site.
- Innate immunity
- The fast, nonspecific first line of defense present from birth, including skin barriers, phagocytes, and inflammation, that attacks any invader without needing prior exposure.
- Pathogen
- Any disease-causing microorganism, such as a bacterium, virus, fungus, or parasite, that can invade the body and trigger an immune response.
- Phagocyte
- A "cell-eating" white blood cell, such as a neutrophil or macrophage, that engulfs and digests germs, dead cells, and debris to clear infection.
DrillFlashcards for this unit
These are the Week 2 cards — the same deck as the hub, pulled in here so you can review the moment you finish reading. Tap the card to flip it; hit Shuffle to mix them up. To grade yourself and track mastery across the whole deck, open the full flashcards page.