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Blood Bank

Structure

Understanding immunoglobulinstheir structure is absolutely fundamental because how they are built directly dictates what they can do – like binding specific antigens on red cells, activating complement, or crossing the placenta

Think of the basic immunoglobulin molecule as a flexible, Y-shaped protein. This shape isn’t accidental; it’s perfectly designed for its dual roles: recognizing and binding a specific target (antigen) and then signaling to other parts of the immune system to take action

Here’s a breakdown of the key structural components:

Basic Building Blocks: Polypeptide Chains

Every basic immunoglobulin unit (monomer) is built from four polypeptide chains:

  • Two Identical Heavy (H) Chains: These are the larger chains (~50-77 kDa each). They form the backbone of the “Y” shape, including the entire stem and part of the arms
    • The type of Heavy chain determines the class (isotype) of the immunoglobulin:
      • Gamma (γ) -> IgG
      • Mu (μ) -> IgM
      • Alpha (α) -> IgA
      • Delta (δ) -> IgD
      • Epsilon (ε) -> IgE
  • Two Identical Light (L) Chains: These are smaller chains (~25 kDa each). Each light chain is bound to one of the heavy chains, forming the outer part of the “Y” arms
    • There are two types (isotypes) of Light chains:
      • Kappa (κ)
      • Lambda (λ)
    • Any single antibody molecule will have either two kappa chains or two lambda chains, never a mix. In humans, about 60% of antibodies have kappa chains and 40% have lambda chains

Holding it Together: Disulfide Bonds

These polypeptide chains are held together by strong covalent bonds called disulfide bonds (-S-S-):

  • Inter-chain disulfide bonds: Link the heavy chains to each other (usually in the hinge region) and link each heavy chain to its corresponding light chain
  • Intra-chain disulfide bonds: Occur within each individual heavy and light chain, creating loops. These loops fold into compact, stable structures called domains

Functional Regions: Variable and Constant

Each heavy and light chain can be conceptually divided into two main regions:

  • Variable (V) Region
    • Location: Found at the tips of the “Y” arms (the N-terminal end of both H and L chains)
    • Composition: Consists of one variable domain on the light chain (VL) and one variable domain on the heavy chain (VH)
    • Function: This region is highly variable in its amino acid sequence, especially within specific segments called Hypervariable Regions or Complementarity-Determining Regions (CDRs). This variability creates the unique, three-dimensional antigen-binding site. Each basic antibody unit has two identical antigen-binding sites, one at the tip of each arm. This is where the antibody recognizes and physically binds to its specific antigen (like the D antigen on a red cell). The incredible diversity here is generated by V(D)J gene rearrangement
  • Constant (C) Region
    • Location: Makes up the rest of the light chain and the stem and lower part of the arms of the heavy chains (the C-terminal end)
    • Composition: Consists of one constant domain on the light chain (CL) and multiple constant domains on the heavy chain (CH1, CH2, CH3 - and sometimes CH4 in IgM and IgE)
    • Function: The amino acid sequence in this region is relatively constant within each class and subclass. It determines the antibody’s class (IgG, IgM, etc.) and mediates its effector functions – how the antibody interacts with other immune system components after binding antigen. This includes:
      • Binding to Fc receptors on cells like macrophages, neutrophils, NK cells, and mast cells
      • Activating the complement system (primarily via the CH2 domain of IgG and CH3 of IgM)
      • Determining if the antibody can cross the placenta (mediated by Fc receptors on placental cells binding the IgG Fc region)

Key Structural Domains

As mentioned, the chains fold into distinct globular domains:

  • Light Chain: VL, CL
  • Heavy Chain: VH, CH1, CH2, CH3 (and CH4 for IgM/IgE)

Each domain has a characteristic “immunoglobulin fold” structure

Functional Fragments: Fab and Fc

Experimentally, enzymes like papain can cleave the antibody molecule into distinct fragments, which helps us understand its function:

  • Fab (Fragment, antigen-binding)
    • Structure: Consists of one complete light chain (VL + CL) paired with the VH and CH1 domains of one heavy chain. Essentially, one “arm” of the Y
    • Function: Contains the antigen-binding site. Each monomeric antibody has two Fab fragments
  • Fc (Fragment, crystallizable)
    • Structure: Consists of the constant region domains from the stem of the two heavy chains (CH2 and CH3 domains, or CH2, CH3, CH4 for IgM/IgE)
    • Function: Contains the sites responsible for most effector functions (complement activation, Fc receptor binding, placental transfer). It’s the “business end” that interacts with the rest of the immune system

The Hinge Region

  • Location: Found between the CH1 and CH2 domains of the heavy chains in IgG, IgA, and IgD
  • Structure: A flexible stretch of amino acids, rich in proline and cysteine residues (which form the inter-chain disulfide bonds holding the heavy chains together here)
  • Function: Allows the two Fab arms to move independently, like hinges on a door. This flexibility is important for allowing the antibody to bind effectively to antigens that might be spaced at different distances on a cell surface or pathogen
  • Note: IgM and IgE lack a hinge region but have an extra constant domain (CH4) instead, which may provide some structural spacing

Carbohydrates (Glycosylation)

Immunoglobulins are glycoproteins, meaning they have carbohydrate chains attached, typically to the constant regions (especially CH2). These carbohydrates are important for:

  • Proper folding and assembly of the antibody
  • Solubility
  • Stability and protection from degradation
  • Modulating Fc-mediated effector functions (influencing binding to Fc receptors and complement)

Overall Structures (Relating to Classes)

While the basic Y-shape is the monomer unit, some classes exist as polymers:

  • Monomer: IgG, IgD, IgE (and membrane-bound IgM/IgA) - The basic 4-chain (2H+2L) structure
  • Dimer: Secretory IgA - Two monomer units linked by a J-chain and associated with a Secretory Component
  • Pentamer: IgM (secreted form) - Five monomer units linked by disulfide bonds and a central J-chain. This structure gives IgM 10 potential antigen-binding sites (though usually only 5 are readily available due to steric hindrance) and makes it exceptionally good at agglutination and complement activation

Blood Bank Relevance Summary

  • Antigen Binding (Fab): The specificity of the V regions dictates which blood group antigen (e.g., D, K, Fya) an antibody will bind
  • Agglutination: The structure (monomer vs. pentamer) influences agglutination ability. Pentameric IgM is a much better agglutinator (visible clumping) than monomeric IgG
  • Complement Activation (Fc): The structure of the Fc region (specifically CH2/CH3) determines if and how well an antibody activates complement (IgM > IgG3 > IgG1). This relates directly to the potential for intravascular hemolysis
  • Placental Transfer (Fc): The IgG Fc structure allows it to bind FcRn receptors and cross the placenta, leading to HDFN if the antibody targets fetal antigens
  • Detection Methods (Overall Structure): Small monomeric IgG often requires enhancement techniques (like the Antiglobulin test using anti-IgG) to be detected, whereas large IgM can often cause direct agglutination
  • Extravascular Hemolysis (Fc): The Fc region of IgG binds to Fc receptors on macrophages in the spleen and liver, leading to the removal of antibody-coated red cells

Key Terms

  • Heavy Chain (H Chain): The larger polypeptide chain in an antibody (~50-77 kDa); its constant region determines the antibody’s class (isotype: γ, μ, α, δ, ε)
  • Light Chain (L Chain): The smaller polypeptide chain in an antibody (~25 kDa); exists as either kappa (κ) or lambda (λ) isotype
  • Disulfide Bonds: Covalent bonds (-S-S-) linking cysteine amino acids; crucial for holding the H and L chains together (inter-chain) and maintaining the folded structure of domains (intra-chain)
  • Variable Region (V Region): The N-terminal portion of H and L chains (VH and VL domains) characterized by high amino acid sequence variability; forms the antigen-binding site
  • Constant Region (C Region): The C-terminal portion of H and L chains (CL, CH1, CH2, CH3, CH4 domains) with relatively stable amino acid sequences within a class; mediates effector functions
  • Domain: A distinct, compact globular structural unit within a polypeptide chain, formed by intra-chain disulfide bonds (e.g., VL, CL, VH, CH1, CH2, CH3)
  • Fab Fragment (Fragment, antigen-binding): One arm of the antibody “Y” (composed of VL, CL, VH, CH1), containing one antigen-binding site
  • Fc Fragment (Fragment, crystallizable): The stem of the antibody “Y” (composed of CH2 and CH3 domains, +/- CH4), responsible for effector functions like complement activation and binding to cellular Fc receptors
  • Hinge Region: A flexible segment between the CH1 and CH2 domains in IgG, IgA, and IgD, allowing movement of the Fab arms
  • J-Chain: A polypeptide that links monomers together in polymeric immunoglobulins (IgM pentamer, IgA dimer)
  • Antigen-Binding Site: The specific pocket or surface within the Variable regions (formed by VH and VL) that physically interacts with and binds the antigen epitope. Each basic antibody monomer has two identical sites