The antigens have a specific shape that determines with which white blood cell or antibody it will interact with. What other molecule's form determines its function?

The correct answer is: the antibodies' shape.

Explanation:

Antibodies are proteins called immunoglobulins that serve the immune system by responding to a specific antigen and generating the proper counter to fight it.

Each antibody consists of four polypeptides that form a "Y" shaped molecule. The Y shape is given by two heavy chains and two light chains.

The tips of the Y are composed of 110-130 amino acids that vary greatly among other antibodies. This gives the antibody its specificity.