Final answer:
The part of an immunoglobulin molecule that exists as kappa or lambda chains is the light chain.
Step-by-step explanation:
The kappa or lambda chains in an immunoglobulin molecule exist as part of the light chain. Immunoglobulins are composed of two identical heavy chains and two identical light chains, and it is these light chains that can be of either kappa (k) or lambda (λ) type. Each immunoglobulin molecule can only have light chains of one type, not both. For instance, in humans, about 65% of light chains are kappa and 35% are lambda.