Antibodies are composed of two types of peptides, heavy chains and light chains. One of the major classes of antibody molecules is immunoglobulin G (IgG). Two heavy chains and two light chains form one IgG molecule. Classify each statement as describing the heavy chains, light chains, or both chains of IgG

a. form the antigen-binding site
b. contain multiple variable domains
c. are found in the Fab fragment
d. are found in the FC fragment
e. contain only one constant domain
f. contain multiple constant domains
g. contain only one variable domain

The antigen-binding site is formed by one hypervariable region of the light chain (VL) and one hypervariable region of the heavy chain (VH). So, both type of chains contributes to antigen recognition.

b. contain multiple variable domains ⇒ none

Both chains - light and heavy - contain only one variable domain (VL and VH).

c. are found in the Fab fragment ⇒ both chains

The Fab fragment is composed of portions of both light and heavy chains: each portion contains one variable domain and one constant domain. The Fab fragment binds specifically the antigen.

d. are found in the FC fragment ⇒ heavy chains

The Fc region is composed of portions of heavy chains (CH² and CH³), so it contains constant domains.

e. contain only one constant domain ⇒ light chains

Each light chain contains one variable domain (VL) and one constant domain (CL).

f. contain multiple constant domains ⇒ heavy chains

Each heavy chain contains three constant domains: CH¹, CH² and CH³.

g. contain only one variable domain ⇒ both chains

Both light and heavy chains contain one variable domain. The difference is given by the number of constant domains: light chains have 1 constant domain while heavy chains have 3 constant domains.