Final answer:
The blank spaces should be filled with "constant" and "variable" respectively; the constant region is universally similar among antigen receptors, while the variable region provides the unique specificity for recognizing antigens.
Step-by-step explanation:
Receptors have two regions. The constant region is the same for all antigen receptors, while the variable region is specific for an antigen. In the antigen receptors found on T cells, such as the T cell receptor (TCR), and on B cells, like B-cell receptors (BCRs), these regions perform different functions. The variable region domain is responsible for recognizing specific antigens due to differences in amino acid sequences that provide the receptor with its unique antigenic specificity. Conversely, the constant region domain shows less variability and is involved in common functions across different receptors such as signal transduction for BCRs. Each T cell produces only one type of receptor, making it specific for a single particular antigen, and this specificity is due to the unique combination of amino acids in the variable region.