Basic IgA structure: In its monomeric serum form, each IgA antibody molecule consists of how many polypeptide chains and how many antigen-binding sites?

Introduction / Context:
Immunoglobulin monomers (IgG, IgD, IgE, and serum IgA) share a common architecture: two identical heavy chains and two identical light chains forming a Y-shaped molecule with two antigen-binding sites.

Given Data / Assumptions:

• We refer to the basic monomeric IgA unit found in serum.
• Secretory IgA in mucosa is commonly a dimer, but the question asks per single antibody molecule.

Concept / Approach:
Each IgA monomer consists of 2 heavy (alpha) chains and 2 light chains (kappa or lambda). The variable regions at the tips of the two Fab arms provide two identical antigen-binding sites (bivalency).

Step-by-Step Solution:
Identify chain composition → 2 heavy + 2 light = 4 polypeptide chains. Determine binding capacity → one site at each Fab arm = 2 sites total. Account for secretory forms separately (dimer with J chain), but answer for a monomer. Select the matching option accordingly.

Verification / Alternative check:
Structural analyses and immunology texts depict Ig monomers as heterotetramers with two combining sites; IgA heavy chains define the class.

Why Other Options Are Wrong:
Two or three chains do not describe immunoglobulin assembly; one binding site would be a Fab fragment, not whole IgA; six chains would imply multimeric Ig beyond a single monomer.

Common Pitfalls:
Confusing secretory dimeric IgA with the monomeric structural unit; mixing Fab fragments with whole antibodies.

Final Answer:
Four polypeptide chains and two antigen-binding sites.