Biology · Introductory biology · Worked example
Count antigen-binding sites
An IgG molecule has two heavy and two light chains. How many antigen-binding sites does it have, and how many do pentameric IgM and secreted, dimeric IgA have? A lab cuts IgG with the enzyme papain into two Fab fragments, the arms, and one Fc fragment, the stem. How many binding sites does each Fab fragment have, and can Fab fragments clump antigens together?
Find the sites on one Y
Each binding site is formed by the variable regions of one heavy and one light chain. IgG has two of each chain, so it has two sites, one at each arm tip, and both bind the same epitope.
Scale up to IgM and IgA
IgM is five Y units joined at their stems, and secreted IgA is two.
Cut with papain
Papain cuts each heavy chain near the hinge, freeing each arm as a Fab fragment, a light chain with part of a heavy chain, and leaving the stem as Fc. Each Fab keeps one complete binding site; the Fc fragment has none.
Ask what clumping needs
To clump antigens, one molecule must hold two antigens at once. A Fab fragment has a single site, so it can bind and cover an epitope, for example to neutralize a toxin, but it cannot link two antigens together.
Result
IgG has 2 identical binding sites, IgM 10 and secreted IgA 4. Each Fab fragment has 1 site, so it can bind and block an epitope but cannot cross-link antigens into clumps.
Your turn
An IgM molecule meets a virus whose surface carries many copies of one epitope. What is the greatest number of copies the IgM molecule can bind at once?
Show the answer and explanation
10.
Five Y units with two binding sites each give 10 sites, and every site binds the same epitope, so all 10 can hold copies of it on the virus.
Keep exploring
The antibody opens in Biology with its chains and regions labelled and an antigen held at each binding site.
Return to the concept →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Try in the workspace
Open the example inputs, change a value and keep a useful result on your board.
Open the antibody Open worked example on a board Immune structures in Biology ReferenceYour existing work stays on this device. Examples open as editable copies.