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Blood Bank

Properties

Understanding the biologic and physical properties of immunoglobulins is key because they explain why different antibody classes behave the way they do in the body and in our test tubes. It’s the link between their structure and their function, which directly impacts things like hemolysis, placental transfer, and how we detect them in the lab

Think of it like this: knowing a tool’s size, shape, and material (physical properties) helps you understand what jobs it’s good for (biologic properties). Same idea with antibodies!

Here’s a breakdown by class, focusing on the properties most relevant to us:

Immunoglobulin G (IgG)

  • Physical Properties
    • Structure: Monomer (basic Y-shape)
    • Size/MW: Relatively small (~150 kDa)
    • Sedimentation Coefficient: 7S (a measure related to size and shape)
    • Stability: Generally stable; can withstand some temperature variations
  • Biologic Properties
    • Concentration: Highest concentration in serum
    • Location: Found in blood, lymph, and tissue fluids (can easily move between them due to size)
    • Antigen Binding: Binds antigen via Fab regions. Usually has high affinity, especially in secondary responses
    • Agglutination: Generally poor agglutinator on its own (“incomplete antibody”). Its monomeric structure means the two binding sites often can’t bridge the distance between red cells suspended in saline (due to the cells’ negative surface charge creating repulsion - the zeta potential). Requires enhancement techniques (LISS, PEG) or antiglobulin reagent (anti-IgG) for detection in vitro
    • Complement Activation: Can activate the classical complement pathway, but usually requires two IgG molecules to bind close together on the cell surface. Subclasses differ: IgG3 > IgG1 >> IgG2 (IgG4 does not activate classical pathway). This can lead to extravascular or sometimes intravascular hemolysis
    • Opsonization: The Fc region binds effectively to Fc receptors (FcγR) on phagocytes (macrophages, neutrophils). This coating (opsonization) flags antibody-coated cells (like red cells) for destruction, primarily in the spleen and liver (extravascular hemolysis)
    • Placental Transfer: Actively transported across the placenta via Fc receptors (FcRn) on placental cells. This is unique to IgG! Provides passive immunity to the fetus/newborn but is also the mechanism for HDFN. Subclasses vary slightly (IgG1/IgG3 cross best)
    • Half-life: Longest half-life (~23 days), providing sustained protection

Immunoglobulin M (IgM)

  • Physical Properties
    • Structure: Large Pentamer (five Y-units joined by J-chain)
    • Size/MW: Very large (~900 kDa)
    • Sedimentation Coefficient: 19S
    • Stability: Disulfide bonds holding the pentamer together can be broken by reducing agents (like DTT or 2-ME), which can be useful in the lab to differentiate IgM from IgG activity
  • Biologic Properties
    • Concentration: Lower than IgG in serum
    • Location: Mostly confined to the intravascular space (bloodstream) due to its large size
    • Antigen Binding: Pentameric structure gives it 10 potential antigen-binding sites (high valency). Even if individual site affinity is low (common in primary response), the overall binding strength (avidity) can be high due to multiple attachments
    • Agglutination: Excellent agglutinator (“complete antibody”). Its large size and multiple binding sites can easily bridge the distance between red cells, causing visible clumping even in saline, often at room temperature or colder
    • Complement Activation: Most efficient activator of the classical complement pathway. The pentameric structure allows the Fc regions to readily bind C1q, initiating the cascade. This can lead to potent intravascular hemolysis
    • Opsonization: Generally less effective at direct opsonization via Fc receptors compared to IgG, but potent complement activation leads to C3b coating, which is strongly opsonic (via complement receptors on phagocytes)
    • Placental Transfer: Does NOT cross the placenta (too large). Therefore, maternal IgM antibodies do not cause HDFN
    • Half-life: Shortest half-life (~5 days)

Immunoglobulin A (IgA)

  • Physical Properties
    • Structure: Monomer in serum; Dimer (with J-chain and Secretory Component) in secretions
    • Size/MW: Monomer ~160 kDa (7S); Dimer ~385 kDa (11S)
  • Biologic Properties
    • Location: Predominant Ig in mucosal secretions (tears, saliva, gut, respiratory tract, milk). Also present in serum
    • Transport: Dimeric IgA is actively transported across epithelial cells into secretions, facilitated by the Secretory Component (which also protects it from digestion)
    • Function: Mucosal immunity - prevents pathogen attachment (immune exclusion). Neutralizes toxins locally
    • Complement Activation: Does NOT activate the classical pathway; may activate the alternative pathway weakly
    • Placental Transfer: Does NOT cross the placenta
    • Blood Bank Relevance: Primarily related to IgA deficiency. IgA-deficient patients can make anti-IgA (usually IgG class) which can cause severe anaphylactic transfusion reactions upon exposure to IgA in blood products

Immunoglobulin E (IgE)

  • Physical Properties
    • Structure: Monomer
    • Size/MW: Slightly larger than IgG (~190 kDa, 8S)
    • Stability: Most heat-labile Ig
  • Biologic Properties
    • Concentration: Lowest concentration in serum
    • Location: Most IgE is tightly bound to Fc receptors (FcεRI) on mast cells and basophils in tissues
    • Function: Triggers allergic reactions. When antigen (allergen) cross-links IgE on mast cells, it causes degranulation and release of histamine and other inflammatory mediators. Also involved in defense against parasites
    • Complement Activation: Does NOT activate complement
    • Placental Transfer: Does NOT cross the placenta
    • Blood Bank Relevance: Responsible for allergic transfusion reactions (urticaria/hives); severe anaphylaxis is possible but less common than with anti-IgA

Immunoglobulin D (IgD)

  • Physical Properties
    • Structure: Monomer
    • Size/MW: ~185 kDa, 7S
  • Biologic Properties
    • Concentration: Very low in serum
    • Location: Primarily found on the surface of naive B lymphocytes (along with IgM) as a B cell receptor (BCR)
    • Function: Involved in signaling for B cell activation and differentiation. Serum function unclear
    • Complement Activation: Does NOT activate complement
    • Placental Transfer: Does NOT cross the placenta
    • Blood Bank Relevance: No known direct role in transfusion reactions or testing

Summary of Key Properties & Blood Bank Implications

Property IgG IgM Blood Bank Significance
Structure Monomer Pentamer IgG needs AHG for detection; IgM often agglutinates directly.
Size Smaller (~150 kDa) Very Large (~900 kDa) IgG moves between blood/tissues; IgM stays intravascular. IgG crosses placenta, IgM doesn’t.
Placental Transfer YES NO IgG causes HDFN; IgM does not.
Complement (Classic) YES (esp. IgG3/IgG1) YES (Very Efficient) Both can cause hemolysis (IgM often intravascular; IgG often extravascular).
Agglutination (RBCs) Poor (“Incomplete”) Good (“Complete”) Affects detection methods (AHG vs. Immediate Spin/RT).
Optimal Temp Usually 37°C (Warm) Often <37°C (Cold), some 37°C (e.g., ABO) Guides testing temperature (IS, RT, 37°C, AHG phases).
Primary Response Minor component Major component IgM appears first after initial exposure.
Secondary Response Major component Minor component IgG dominates upon re-exposure (anamnestic response).
Half-life Long (~23 days) Short (~5 days) IgG provides longer-lasting immunity/antibody presence.

Key Terms

  • Physical Properties: Characteristics related to the molecule’s size, shape, charge, and stability (e.g., molecular weight, sedimentation coefficient, structure - monomer/pentamer)
  • Biologic Properties: Characteristics related to the molecule’s function in a biological system (e.g., complement activation, placental transfer, opsonization, agglutination, half-life)
  • Monomer: A single basic structural unit of an immunoglobulin (one Y-shape, composed of 2 heavy and 2 light chains). IgG, IgD, IgE are primarily monomers
  • Pentamer: A structure composed of five immunoglobulin monomers linked together, characteristic of secreted IgM
  • Agglutination: The clumping of particles (like red blood cells) due to antibody cross-linking
  • Complement Activation: The triggering of the complement cascade, often initiated by IgM or IgG binding to antigen, leading to inflammation, opsonization, and cell lysis
  • Opsonization: The coating of a particle (e.g., red blood cell) with antibodies (like IgG) or complement fragments (like C3b) that enhances its phagocytosis by cells bearing Fc or complement receptors
  • Placental Transfer: The active transport of IgG across the placenta from mother to fetus
  • Half-life: The time required for the concentration of a substance (like an immunoglobulin) in the body to decrease by half
  • Valency: The number of antigen-binding sites on an antibody molecule (e.g., 2 for monomeric IgG, theoretically 10 for pentameric IgM)
  • Avidity: The overall strength of binding between a multivalent antibody and a multivalent antigen, reflecting the combined strength of all individual binding sites
  • Affinity: The strength of binding between a single antigen-binding site on an antibody and a single epitope on an antigen
  • Sedimentation Coefficient (Svedberg unit, S): A measure of how quickly a molecule sediments in a centrifuge, related to its mass and shape. Larger molecules generally have higher S values (e.g., IgM 19S vs. IgG 7S)