T cell maturation and development of their antigen-specific receptors takes place in the ____________ . B cells mature in the ____________ where they also develop their specificity to a single antigen. This specificity in both B and T cells is made possible through genetic ____________ . The genes that code for the variable regions of both B and T cell receptors are cut and spliced during the ____________ process of both cells. Because genetic rearrangement occurs at the level of the ____________ rather than in the RNA following ____________ , the changes are permanent, resulting in each cell being capable of binding to a single antigen in a pool of up to 1018 possible variations. B cell receptors are ____________ that are comprised of four ____________ and have two identical antigen binding sites. The ____________ region of the immunoglobulin is embedded in the cell membrane, and the variable regions are facing outward in order to bind to their potential antigenic match. Unlike B cell receptors, T cell receptors are comprised of two polypeptides and have ____________ antigen binding site that is exposed. Like B cell receptors, their specificity to a single antigen is determined ____________ to any encounters with antigens.