Lecture 18 - Hypersensitivity Part 2

Hemolytic Disease of the Newborn (HDN)

Type II hypersensitivity, maternal IgG antibodies bind directly to fetal RBC antigens, causing their destruction

• Maternal antibodies vs blood group antigens cross placenta

  • Often vs. Rh antigens

  • Destroy fetal RBC

Type II Hypersensitivity - Hemolytic Disease of the Newborn (HDN)

• Initial birth (Rh-negative mother carries Rh-positive fetus)

  • Fetal RBCs enter mother’s circulation

    • Mother’s antibody response (IgM) to fetal antigens (Rh)

    • IgM don’t cross placenta; first child not affected

• Subsequent birth(s)

  • If fetus possesses same antigens:

    • Memory B cell response; IgG vs Rh crosses placenta

    • Binds and destroys fetal RBCs

  • Possible loss of pregnancy

  • Possible birth with hemolytic disease

    • Treatment = Transfusions

Type II Hypersensitivity - Hemolytic Disease of the Newborn (HDN) Prevention

• Screening during pregnancy

• RhoGAM (anti-Rh IgG Antibodies) given to Rh-negative mothers

  • Given at 28 weeks and again upon delivery

  • Opsonization, clearance of fetal RBCs

    • Prevent maternal immune/memory response

      • RhoGAM coats any fetal Rh+ RBCs that enter maternal blood.

      • These coated RBCs are quickly cleared before the mother’s immune system detects them

Why IgG matters in HDN?

• IgM = stays in mother, cannot hurt fetal RBCs.

• IgG = crosses placenta, coats fetal RBCs, and causes destruction.

This shift from IgM (primary response) → IgG (memory response) is why later pregnancies are at risk.

Type II Hypersensitivity - Autoimmune Hemolytic Anemia (AIHA)

The body makes antibodies against self-antigens on it’s own RBCs, causing RBC destruction. There are two types based on the temperature at which the antibodies bind RBCs

• Warm reactive antibodies (react at 37 degrees C)

• Cold reactive antibodies (react below 30 degrees C)

Warm Autoimmune Hemolytic Anemia

• IgG mediated (IgG antibodies bind RBCs); strong reaction at higher temperature

  • Cause often unknown

  • Associated with some viral infections

  • Associated with some drugs acting as haptens

  • RBC clearance in liver and spleen (opsonization/phagocytosis); hemolysis via complement

Warm Autoimmune Hemolytic Anemia - Treatments

• Corticosteriods - reduce B cell response

• Splenectomy - reduce RBC clearance

Cold Autoimmune Hemolytic Anemia

• Less frequent than warm

• Antibody binding is reversible when the patient warms up

• Transient - few weeks

• IgM-mediated

• Often Secondary to Mycoplasma and some viral infections

• Occasionally unknown origin

Cold Autoimmune Hemolytic Anemia - Mechanism (What happens)

• Effects when peripheral circulation temperature is low

  • IgM binds RBCs = RBC agglutination, capillary blockage; necrosis possible

  • Extremities - Fingers, toes, ears, etc.

• Effects when body temperature warms

  • Complement reactions proceed

  • Opsonization/phagocytosis

  • Hemolysis (destruction/breakdown of RBCs)

Cold Autoimmune Hemolytic Anemia - Prevention

Avoid cold and keep extremities warm

Type II Hypersensitivity - Detection

• Direct Antiglobulin Test (Coombs test)

  • Detect opsonized RBCs

  • To detect whether RBCs in the patient’s body are already coated with:

    • IgG Antibodies

    • Complement Components (Cd3b, Cdd, C4b)

  • RBCs + anti-human antibodies vs IgG, C3b, C3d, C4b

  • Agglutination = positive

• ABO Cross-matching Tests

Direct Antiglobulin Test (Coombs test)

1. Take the patient’s RBCs

• These RBCs may or may not have antibodies/complement stuck to them.

2. Add anti-human antibodies

These are antibodies that bind to:

• Human IgG

• Human complement proteins (C3b, C3d, C4b)

This reagent is sometimes called Coombs reagent.

3. Observe for agglutination

• If the patient’s RBCs are coated with IgG or complement, the added anti-human antibodies will crosslink the RBCs, causing visible clumping.

Result Interpretation

• Agglutination = Positive DAT

  • Means RBCs are coated with IgG or complement → Type II cytotoxic reaction is occurring

Type III Hypersensitivity

• Immune complex-mediated

  • Caused by IgG or IgM antibodies binding to soluble antigens (not on cell surfaces).

• Complexes deposited in tissues

  • Joints

  • Basement membranes- glomeruli

  • Alveolar tissues

• Once deposited, they trigger complement and inflammation.

Type III Hypersensitivity - Complexes Bind Complement

Complement binding leads to:

• Outcomes of complement

  • Vasodilation, vascular permeability

  • Phagocyte recruitment

  • Opsonization

  • Mast cell activation, degranulation;inflammation

• Inability of Neutrophils to phagocytose = damage

  • Release of granules, lysosome contents

  • Tissue damage

    • Long-term: scar tissue, dysfunction

• Mechanism for SLE, RA

Type III hypersensitivity - Arthus Reaction

• A localized immune complex reaction in the skin

  • Classical Example: Occasionally seen in vaccine booster reactions

• Immunization of rabbits: antibody development

• Intradermal Challenge

  • Redness, swelling peaking in hours

  • Necrotic lesion

• Induced by immune complexes

  • In basement membranes of vessels

  • Complement activated

  • Neutrophils and mast cells activated

    • Release of contents

    • Mast cells

Type III hypersensitivity - Serum Sickness

A systemic immune complex disease caused by exposure to foreign proteins (often from animal serum).

• Passive Immunization with animal serum

  • Past: common therapy for some infections

  • Today:

    • Horse anti-venom for snake poison

    • Reaction to mouse monoclonal antibodies

• Development of antibodies specific for the animal proteins

  • Immune complexes in tissue

  • Symptoms = 1-3 weeks

    • General symptoms of illness

    • Joint and kidney effects

Type IV Hypersensitivity

• T-cell mediated

  • Symptoms peak: 48-72 hours later

  • Known as Delayed-Type Hypersensitivity (DTH)

  • Antigens (that trigger it)

    • Intracellular infections, insect venom

    • Contact antigens

      • E.g., poison ivy, metal

Type IV Hypersensitivity Mechanism

1.) Initial (first) exposure

• Initial Antigen response: Th1 cell-mediated

  • Langerhans cells frequently present Antigen

  • Th1 response at the site

    • Th1 cells become memory cells and no symptoms yet

2.) Subsequent exposure - Effector (Reaction) Phase

• Subsequent response: Memory T cell activation

  • Keratinocytes activated, produce chemokines

  • Monocytes recruited; macrophages in tissues

    • Macrophage Activation

    • Release Reactive oxygen species (ROS) and inflammatory cytokines

• Sometimes the reaction can be CD8+ T cell-mediated instead of Th1 cells

  • This depends on Antigen-processing pathway

Type IV Hypersensitivity - Contact Dermatitis

Triggered by low molecular-weight compounds that penetrate the skin

• These chemicals acts as haptens

Common Triggers:

• Urushiol from poison ivy and poison oak

• Other chemicals such as nickel

Why is Contact Dermatitis a Type IV reaction?

• It has a long-lasting memory T cell response

  • Causes a delayed reaction = 6 hours to several days later at the site of exposure

Symptoms of Contact Dermatitis

• Redness, swelling, itchiness

• Papules; bilstering

• Which can last for days to weeks

TB Skin Test (PPD)

You are checking whether the person has memory T cells specific for Mycobacterium tuberculosis.

Type IV Hypersensitivity - TB Skin Test (PPD)

• Antigen = Purified Protein Derivative (PPD) of Mycobacterium tuberculosis

• Tests T cell memory

  • Current TB

  • Previous exposure to TB

  • Vaccination for TB

How TB Skin Test (PPD) works

• Intradermal injection of Antigen (PPD) (under the skin)

  • Wait 2-3 days later

    • Hard, raised lesion (Induration) = POSITIVE

    • Diameter is measured

    • Different criteria for positivity (diameter)

TB Skin Test (PPD) - Criteria for positivity depend on risk

• No/Low risk factors (healthy person)

• Higher risk groups:

  • People from TB-endemic regions

  • Occupational risk (healthcare workers)

  • Immunospression (HIV, transplant, etc.)

  • IV drug users