5. Immunopathology

Learning Objectives

• Classify hypersensitivity reactions into Types I-IV and explain the mechanism driving each
• Describe the pathophysiology of anaphylaxis and other Type I reactions
• Distinguish autoimmune diseases by the pattern of self-tolerance breakdown, using SLE and rheumatoid arthritis as models
• Differentiate primary (congenital) from secondary (acquired) immunodeficiency and give examples of each
• Explain the immunological basis of transplant rejection and the timeline over which each type occurs
• Apply hypersensitivity classification to clinical scenarios such as blood transfusion reactions and contact dermatitis

Quick Answer

Immunopathology is the study of disease caused by the immune system itself — either overreacting, misdirecting its attack, or failing to function. It has four pillars: hypersensitivity reactions (Gell and Coombs Types I-IV, ranging from allergic anaphylaxis to delayed-type contact dermatitis), autoimmune diseases (where tolerance to self-antigens breaks down, as in lupus and rheumatoid arthritis), immunodeficiency (primary genetic defects or secondary causes like HIV and chemotherapy that leave the body vulnerable to infection), and transplant rejection (the immune system recognizing donor tissue as foreign). Understanding these four categories explains why the same immune system that protects us can also cause some of the most serious diseases in medicine.

Overview

Every immune response is a trade-off. A system powerful enough to kill bacteria, viruses, and cancer cells is also powerful enough to damage the host if it is triggered inappropriately, aimed at the wrong target, or missing altogether. Immunopathology is the branch of pathology that studies exactly when and how that goes wrong.

Think of the immune system as having four possible failure modes, and each one maps onto a major disease category you will be tested on repeatedly through medical school and clinical practice:

1. Too strong, wrong trigger — hypersensitivity reactions (allergies, autoimmune-mediated tissue damage, serum sickness, contact dermatitis)
2. Too strong, wrong target — autoimmune disease (attacking self instead of foreign antigen)
3. Too weak — immunodeficiency (unable to mount an adequate response)
4. Correctly identifying "foreign" when it shouldn't matter — transplant rejection (attacking donor tissue that is technically foreign but therapeutically necessary)

The unifying thread across all four is antigen recognition. Nearly everything in this chapter comes back to a single question: what does the immune system think it is looking at, and what does it do once it decides?

Hypersensitivity Reactions (Gell and Coombs Classification)

Definition

Hypersensitivity is an exaggerated or inappropriate immune response that causes tissue damage. The Gell and Coombs system divides these reactions into four types based on the immune mechanism involved — this classification is one of the most heavily tested concepts in pathology.

Explanation

The four types differ in three key ways: which immune component does the damage (antibody vs. T cell), how fast the reaction occurs, and what triggers it.

Type I — Immediate (IgE-mediated) hypersensitivity On first exposure to an allergen, B cells are class-switched to produce IgE, which binds to Fc receptors on mast cells and basophils — this is sensitization, and it causes no symptoms. On re-exposure, the allergen cross-links adjacent IgE molecules on the mast cell surface, triggering degranulation within minutes. Released histamine, leukotrienes, and prostaglandins cause vasodilation, increased vascular permeability, smooth muscle contraction, and mucus secretion. Examples: anaphylaxis, allergic rhinitis (hay fever), asthma, urticaria, food allergy.

Type II — Antibody-mediated (cytotoxic) hypersensitivity IgG or IgM antibodies bind directly to antigens on the surface of a cell or in the extracellular matrix, triggering destruction through three routes: complement activation (membrane attack complex), antibody-dependent cellular cytotoxicity (NK cells and macrophages via Fc receptors), or opsonization leading to phagocytosis. Examples: ABO blood transfusion reactions, hemolytic disease of the newborn (Rh incompatibility), Goodpasture syndrome (anti-glomerular basement membrane antibodies), myasthenia gravis (anti-acetylcholine receptor antibodies), Graves disease (stimulating TSH receptor antibodies — technically a variant since it activates rather than destroys the target).

Type III — Immune complex-mediated hypersensitivity Antigen-antibody complexes form in the circulation and deposit in blood vessel walls, joints, kidneys, and skin rather than at the site of antigen entry. Deposited complexes activate complement, attracting neutrophils that release lysosomal enzymes and cause tissue damage — this typically takes hours to days. Examples: serum sickness, systemic lupus erythematosus (immune complexes deposit in the glomerular basement membrane), post-streptococcal glomerulonephritis, the Arthus reaction.

Type IV — Delayed-type (cell-mediated) hypersensitivity No antibody is involved. Sensitized CD4+ T helper cells (Th1) release cytokines that recruit and activate macrophages, or CD8+ cytotoxic T cells directly kill target cells. Because it depends on T cell trafficking and cytokine signaling rather than pre-formed antibody, onset is delayed 48-72 hours. Examples: the tuberculin (Mantoux) skin test, contact dermatitis (poison ivy, nickel allergy), graft-versus-host disease, and the chronic tissue damage seen in tuberculosis granulomas.

Example

A child eats peanuts for the first time with no reaction (sensitization occurs silently). Months later, a small amount of peanut protein triggers lip swelling, hives, and wheezing within minutes — classic Type I.

Real-World Example

A patient receives a mismatched blood transfusion. Recipient antibodies against donor RBC surface antigens trigger immediate complement-mediated hemolysis, fever, back pain, and hemoglobinuria — Type II in real time, and the reason blood banks perform meticulous cross-matching before every transfusion.

Why It Matters

The Gell and Coombs classification isn't just an academic exercise — it predicts management. Type I anaphylaxis is treated with epinephrine within minutes because histamine-driven laryngeal edema and shock kill quickly. Type IV contact dermatitis is treated with topical steroids over days because the T-cell response builds slowly and resolves slowly. Knowing the type tells you the timeline and the treatment.

Common Misunderstanding

Students often assume all "allergic" reactions are Type I. Graves disease and myasthenia gravis are antibody-mediated but are Type II, not Type I, because the antibody targets a cell-surface receptor directly rather than triggering IgE-mast cell degranulation. Similarly, SLE is frequently mislabeled as purely autoimmune without recognizing that its tissue damage (especially the kidney disease) is mechanistically a Type III immune-complex reaction.

Autoimmune Diseases

Definition

Autoimmune disease occurs when central or peripheral tolerance mechanisms fail, and the immune system generates antibodies or T cells against self-antigens, causing chronic tissue damage.

Explanation

Normally, T cells that strongly recognize self-antigens are deleted in the thymus (central tolerance) or suppressed by regulatory T cells in the periphery (peripheral tolerance). Autoimmunity develops when this fails — through genetic susceptibility (certain HLA alleles), molecular mimicry (a pathogen antigen resembling a self-antigen, as in rheumatic fever following streptococcal infection), failure of regulatory T cells, or exposure of previously "hidden" self-antigens after tissue injury.

Systemic lupus erythematosus (SLE) is the classic multisystem autoimmune disease and a favorite exam topic. Anti-nuclear antibodies (ANA) are highly sensitive but not specific; anti-double-stranded DNA and anti-Smith antibodies are far more specific for SLE. Immune complexes of these antibodies with nuclear antigens deposit in the skin (malar "butterfly" rash), joints (non-erosive arthritis), kidneys (lupus nephritis — the major cause of mortality), and serosal surfaces (pleuritis, pericarditis). This is a Type III mechanism layered on top of a fundamentally autoimmune process.

Rheumatoid arthritis targets the synovium of joints; rheumatoid factor (anti-IgG antibody) and anti-cyclic citrullinated peptide (anti-CCP) antibodies drive chronic synovitis, pannus formation, and joint destruction — anti-CCP is more specific.

Type 1 diabetes mellitus results from T-cell-mediated (Type IV-like) destruction of pancreatic beta cells, usually with autoantibodies against insulin and glutamic acid decarboxylase (GAD) as markers rather than direct effectors.

Example

A young woman develops a facial rash worsened by sunlight, joint pain in both hands, and unexplained proteinuria. Positive ANA and anti-dsDNA point strongly toward SLE.

Real-World Example

Rheumatic fever after untreated streptococcal pharyngitis: antibodies against the bacterial M protein cross-react with cardiac myosin (molecular mimicry), causing carditis and valve damage weeks after the throat infection has resolved.

Why It Matters

Autoimmune diseases are chronic, often relapsing-remitting, and disproportionately affect women — recognizing the pattern early (which antibody, which organ, which demographic) changes prognosis, since untreated lupus nephritis or rheumatoid joint destruction cause irreversible organ and joint damage.

Common Misunderstanding

A positive ANA does not mean a patient has lupus — ANA is present in up to 5% of healthy people and in many other autoimmune and even infectious conditions. It is a sensitive screening test, not a diagnostic one; the specific autoantibody pattern and clinical criteria establish the diagnosis.

Immunodeficiency

Definition

Immunodeficiency is a state in which one or more components of the immune system are absent or functioning inadequately, leading to increased susceptibility to infection.

Explanation

Immunodeficiencies are divided into primary (congenital), caused by inherited genetic defects usually presenting in infancy or early childhood, and secondary (acquired), caused by an external factor affecting a previously normal immune system.

Key primary immunodeficiencies, organized by the arm of immunity affected:

• B-cell defects (humoral): X-linked (Bruton) agammaglobulinemia — a BTK gene mutation blocks B-cell maturation, causing recurrent bacterial (encapsulated organism) infections after maternal antibodies wane around 6 months of age. Common variable immunodeficiency (CVID) presents later in life with recurrent sinopulmonary infections.
• T-cell defects: DiGeorge syndrome — thymic aplasia from a 22q11 deletion causes T-cell deficiency along with cardiac defects, hypocalcemia (absent parathyroids), and characteristic facies.
• Combined B- and T-cell defects: Severe combined immunodeficiency (SCID) — multiple genetic causes (e.g., adenosine deaminase deficiency) essentially abolish adaptive immunity; presents with severe, recurrent infections by all organism types (bacterial, viral, fungal, opportunistic) in the first months of life and is fatal without bone marrow transplant.
• Phagocyte defects: Chronic granulomatous disease — NADPH oxidase deficiency prevents the oxidative burst needed to kill catalase-positive organisms (Staph aureus, Aspergillus, Serratia), causing recurrent abscesses and granulomas.
• Complement defects: C5-C9 (membrane attack complex) deficiency causes recurrent Neisseria infections specifically.

Secondary immunodeficiency is far more common clinically: HIV infection (destroys CD4+ T cells, defining AIDS at a count below 200 cells/microliter, with characteristic opportunistic infections like Pneumocystis jirovecii pneumonia and CMV retinitis), immunosuppressive medications, chemotherapy, malnutrition, and diabetes.

Example

A 7-month-old boy who was healthy at birth develops recurrent otitis media, pneumonia, and sinusitis from encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae) as maternal IgG wanes — suggestive of X-linked agammaglobulinemia.

Real-World Example

A patient with untreated HIV develops oral thrush, unintentional weight loss, and eventually Pneumocystis pneumonia as the CD4 count falls below 200 — the clinical hallmark of progression to AIDS, and the rationale for prophylactic antibiotics once the count drops that low.

Why It Matters

Recognizing the pattern of infection tells you which arm of immunity is broken: recurrent bacterial infections point to antibody or complement deficiency, while recurrent viral, fungal, and opportunistic infections point to T-cell deficiency. This pattern recognition guides both diagnosis and empiric prophylaxis.

Common Misunderstanding

Students often assume immunodeficiency always means "no immune response at all." In reality, most primary immunodeficiencies are selective — a defect in one arm (say, B cells) leaves other arms (T cells, phagocytes) intact, which is exactly why the infection pattern is diagnostically useful rather than uniformly severe.

Transplant Rejection

Definition

Transplant rejection is an immune response against donor tissue whose surface antigens (primarily HLA/MHC molecules) are recognized as foreign.

Explanation

Rejection is classified by timing, which reflects the underlying mechanism:

• Hyperacute rejection (minutes to hours): pre-formed recipient antibodies against donor antigens (from prior transfusions, pregnancy, or transplants) bind immediately, activating complement and causing thrombosis of graft vessels — a Type II mechanism. This is why cross-matching is done before transplantation, and it is now rare in practice.
• Acute rejection (weeks to months): cell-mediated (host cytotoxic T cells attacking donor cells directly, a Type IV process) and/or antibody-mediated. This is the type most amenable to treatment with immunosuppression and is caught on surveillance biopsy.
• Chronic rejection (months to years): a slow process of vascular intimal thickening (graft arteriosclerosis) and fibrosis driven by a low-grade, ongoing immune response combined with repeated subclinical injury; it is largely irreversible and the leading cause of long-term graft failure.
• Graft-versus-host disease (GVHD), distinct from rejection, occurs after bone marrow/stem cell transplant when immunocompetent donor T cells attack the recipient's tissues (skin, gut, liver) — the reverse direction of a normal rejection.

Example

A kidney transplant recipient has stable creatinine for two years, then develops slowly progressive renal dysfunction with biopsy showing vascular fibrosis and no acute inflammation — consistent with chronic rejection.

Real-World Example

A bone marrow transplant patient develops a skin rash, diarrhea, and abnormal liver function tests two weeks after transplant — classic acute GVHD, where donor T cells recognize recipient tissue as foreign.

Why It Matters

The timing classification directly determines management: hyperacute rejection is essentially untreatable once triggered (prevented only by pre-transplant cross-matching), acute rejection responds to increased immunosuppression, and chronic rejection is largely irreversible — which is why transplant medicine focuses heavily on minimizing the risk factors (HLA matching, adherence to immunosuppressants) that lead to chronic injury in the first place.

Common Misunderstanding

Students often think rejection and graft-versus-host disease are the same process in opposite direction conceptually but confuse which cells are attacking whom. In rejection, host immune cells attack the graft. In GVHD, transplanted donor immune cells attack the host. Confusing these leads to backward reasoning about which transplant type (solid organ vs. bone marrow) is at risk for which complication.

Key Terms

Term	Definition	Related Concept
Type I hypersensitivity	IgE-mediated, mast cell degranulation, immediate onset	Anaphylaxis, allergic rhinitis, asthma
Type II hypersensitivity	Antibody bound directly to cell-surface or matrix antigen	Transfusion reaction, Goodpasture syndrome
Type III hypersensitivity	Circulating immune complexes deposit in tissue	SLE, serum sickness, glomerulonephritis
Type IV hypersensitivity	Delayed, T-cell-mediated, no antibody involved	Contact dermatitis, tuberculin test, GVHD
Molecular mimicry	Pathogen antigen resembles self-antigen, triggering autoimmunity	Rheumatic fever, Guillain-Barré syndrome
ANA	Anti-nuclear antibody; sensitive but nonspecific screening test	SLE, mixed connective tissue disease
Anti-dsDNA	Highly specific antibody for SLE, correlates with lupus nephritis activity	SLE, Type III mechanism
SCID	Severe combined immunodeficiency; combined B- and T-cell failure	Bone marrow transplant, ADA deficiency
X-linked agammaglobulinemia	BTK gene defect blocking B-cell maturation	Recurrent encapsulated bacterial infections
DiGeorge syndrome	22q11 deletion causing thymic aplasia and T-cell deficiency	Hypocalcemia, cardiac defects
Hyperacute rejection	Pre-formed antibody-mediated rejection within minutes to hours	Cross-matching, Type II mechanism
Chronic rejection	Irreversible graft vascular fibrosis over months to years	Graft arteriosclerosis, long-term graft loss
GVHD	Graft-versus-host disease; donor immune cells attack recipient	Bone marrow transplant

Common Mistakes

Misconception: All hypersensitivity reactions are "allergies" and happen quickly. Why it's wrong: Only Type I reactions are truly immediate (minutes). Type III reactions take hours, and Type IV reactions are, by definition, delayed 48-72 hours because they depend on T-cell trafficking rather than pre-formed antibody. Correct understanding: Each Gell and Coombs type has a distinct mechanism and timeline — antibody-mediated types (I, II, III) act faster than the T-cell-mediated Type IV.

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Misconception: A positive ANA test confirms a diagnosis of lupus. Why it's wrong: ANA is present in a meaningful percentage of the healthy population and in many other autoimmune, infectious, and even malignant conditions. It is a highly sensitive screening tool, not a specific diagnostic one. Correct understanding: Diagnosis of SLE requires clinical criteria plus more specific serology (anti-dsDNA, anti-Smith) — ANA alone only tells you to look further, not that the diagnosis is confirmed.

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Misconception: Immunodeficiency always means the patient gets every type of infection equally. Why it's wrong: Most immunodeficiencies are selective for one arm of the immune system, so the pattern of infection reflects exactly which arm is missing — recurrent encapsulated bacterial infections suggest antibody or complement deficiency, while recurrent viral, fungal, and opportunistic infections suggest T-cell deficiency. Correct understanding: Use the infection pattern (organism type, age of onset, site) as a diagnostic clue to localize the defect within the immune system, rather than assuming uniform vulnerability.

Comparison and Connections

Feature	Type I	Type II	Type III	Type IV
Mediator	IgE, mast cells	IgG/IgM against cell-surface antigen	Antigen-antibody immune complexes	T cells (no antibody)
Onset	Minutes	Minutes to hours	Hours to days	48-72 hours
Classic example	Anaphylaxis, hay fever	Transfusion reaction, Goodpasture	SLE, serum sickness	Contact dermatitis, TB skin test
Key mechanism	Mast cell degranulation	Complement/ADCC/opsonization	Complement activation after tissue deposition	Macrophage/cytotoxic T-cell activation

Practice Questions

Recall

1. List the four Gell and Coombs hypersensitivity types and name one example disease for each. Guidance: Type I - anaphylaxis; Type II - Goodpasture syndrome; Type III - SLE; Type IV - contact dermatitis.

2. Name three primary immunodeficiencies and the arm of immunity each affects. Guidance: X-linked agammaglobulinemia (B cell), DiGeorge syndrome (T cell), SCID (combined B and T cell), chronic granulomatous disease (phagocyte).

Understanding

3. Explain why Type IV hypersensitivity reactions take 48-72 hours to develop while Type I reactions occur within minutes. Guidance: Type I relies on pre-formed IgE already bound to mast cells, so degranulation is immediate on re-exposure. Type IV requires sensitized T cells to traffic to the tissue and recruit/activate macrophages via cytokines, a slower cellular process.

4. Why is anti-dsDNA a more useful test than ANA for confirming a diagnosis of SLE? Guidance: ANA is sensitive but nonspecific (positive in healthy people and other diseases); anti-dsDNA is much more specific for SLE and correlates with disease activity, particularly lupus nephritis.

Application

5. A patient receives a mismatched blood transfusion and develops fever, flank pain, and hemoglobinuria within an hour. Which hypersensitivity type is responsible and what is the mechanism? Guidance: Type II. Pre-formed recipient antibodies bind donor RBC surface antigens, activating complement and causing intravascular hemolysis.

6. An infant develops recurrent severe infections with bacteria, viruses, and fungi starting at 2 months of age and fails to thrive. What is the most likely diagnosis and definitive treatment? Guidance: Severe combined immunodeficiency (SCID). Definitive treatment is hematopoietic stem cell (bone marrow) transplant; without it the condition is fatal.

Analysis

7. Compare the mechanism of hyperacute versus chronic transplant rejection and explain why one is preventable and the other is not. Guidance: Hyperacute is caused by pre-formed antibodies and is prevented by pre-transplant cross-matching. Chronic rejection results from cumulative, low-grade cell-mediated and humoral injury over years and is largely irreversible once vascular fibrosis develops — it cannot be prevented outright, only its risk minimized through matching and adherence to immunosuppression.

8. A patient with rheumatoid arthritis (Type III/IV-associated autoimmune process) receives immunosuppressive therapy and subsequently develops recurrent bacterial pneumonia. Explain how treating one immunopathologic problem created another. Guidance: Immunosuppressants dampen the autoimmune attack on the synovium but also blunt normal protective immunity, converting the patient from an autoimmune-disease risk profile to a secondary immunodeficiency risk profile — illustrating the therapeutic trade-off central to immunopathology.

FAQ

Why does anaphylaxis kill so quickly compared to other allergic reactions? Anaphylaxis is a systemic Type I reaction: mast cells throughout the body degranulate simultaneously, releasing histamine and other mediators that cause widespread vasodilation (hypotension/shock), laryngeal edema (airway obstruction), and bronchospasm all at once. Epinephrine is given immediately because it reverses all three problems — it vasoconstricts, reduces mucosal edema, and bronchodilates — buying time before the reaction can be brought fully under control.

Why are women affected by autoimmune diseases so much more often than men? The reasons are not fully settled, but leading theories include estrogen's immunostimulatory effects on antibody production, X-chromosome-linked immune genes (women have two X chromosomes, and skewed X-inactivation may expose X-linked self-antigens that escape tolerance), and differences in microchimerism from pregnancy. SLE, for example, affects women roughly 9 times more often than men during reproductive years.

What is the practical difference between primary and secondary immunodeficiency in terms of when to suspect each? Primary immunodeficiencies typically present in infancy or early childhood with recurrent, severe, or unusual infections and a family history. Secondary immunodeficiencies are far more common overall and should be suspected in any patient with a clear precipitating cause — HIV risk factors, chemotherapy, chronic corticosteroid use, malnutrition, or a chronic disease like diabetes — regardless of age.

Can a person have more than one hypersensitivity mechanism operating in the same disease? Yes, and SLE is the textbook example: it is fundamentally an autoimmune disease (loss of tolerance to nuclear antigens) whose tissue damage is mechanistically Type III (immune complex deposition in the kidney, skin, and joints). Recognizing that a single disease can combine categories is important for both exams and understanding treatment targets.

Why is HLA matching so important in transplantation if immunosuppressive drugs exist? Better HLA matching reduces the intensity and frequency of both acute and chronic rejection, meaning lower doses of immunosuppression are needed over the patient's lifetime. Since immunosuppression carries its own risks (infection, malignancy, drug toxicity), good matching is not a substitute for drugs but a way to reduce the cumulative burden of both rejection and its treatment.

Quick Revision

• Gell and Coombs Types I-IV: Type I (IgE, immediate, mast cells), Type II (antibody vs. cell-surface antigen), Type III (immune complexes deposit in tissue), Type IV (T-cell mediated, delayed 48-72h)
• Type I mechanism: sensitization produces IgE bound to mast cells; re-exposure cross-links IgE, triggering histamine release
• Type II examples: transfusion reactions, Goodpasture syndrome, myasthenia gravis, Graves disease
• Type III examples: SLE, serum sickness, post-streptococcal glomerulonephritis
• Type IV examples: TB skin test, contact dermatitis, graft-versus-host disease
• SLE: ANA is sensitive/nonspecific; anti-dsDNA and anti-Smith are specific; nephritis is the major cause of mortality
• Primary immunodeficiencies are selective by immune arm: B cell (agammaglobulinemia), T cell (DiGeorge), combined (SCID), phagocyte (CGD), complement (Neisseria susceptibility)
• HIV/AIDS is the classic secondary immunodeficiency; CD4 count under 200 defines AIDS and opportunistic infection risk
• Infection pattern localizes the immune defect: bacterial → antibody/complement; viral/fungal/opportunistic → T cell
• Transplant rejection timing reflects mechanism: hyperacute (pre-formed antibody, minutes-hours), acute (cell-mediated, weeks-months, treatable), chronic (fibrosis, months-years, irreversible)
• GVHD is the reverse of rejection: donor immune cells attack recipient tissue after bone marrow transplant
• Molecular mimicry (pathogen antigen resembling self) is a key trigger for autoimmunity, exemplified by rheumatic fever

Related Topics

Prerequisites: Basic immunology (innate vs. adaptive immunity, antibody structure, T-cell subsets, complement pathway)

Related Topics: General Pathology (inflammation, cell injury), Rheumatology topics (SLE, rheumatoid arthritis in detail), Microbiology (opportunistic infections in immunodeficiency), Hematology (transfusion medicine)

Next Topics: Neoplasia and Tumor Immunology, Infectious Disease Pathology