Final answer:
The unique antigen-reactive proteins of T cells are T cell receptors (option B), which are membrane-bound proteins used for specific antigen recognition. They differ from antibodies, which can be secreted and float freely in the body. TCRs recognize antigens presented by MHC molecules on antigen-presenting cells, thereby playing a critical role in the adaptive immune response.
Step-by-step explanation:
The unique antigen-reactive proteins of T cells are T cell receptors (TCRs). TCRs are specialized proteins on the surface of T cells that recognize antigen fragments bound to major histocompatibility complex (MHC) molecules. Unlike immunoglobulins (which are secreted by B cells and are also known as antibodies when they are free-floating), TCRs remain membrane-bound and are responsible for the specific recognition of antigens. When an antigen is presented by an antigen-presenting cell, the TCR binds to the antigen-MHC complex, initiating a series of events that lead to T cell activation and the adaptive immune response.
Each T cell receptor is made up of two different polypeptide chains, generally alpha and beta chains, which are responsible for antigen recognition. The diversity of TCRs is generated through a similar genetic rearrangement process to that used by B cells for their B-cell receptors (BCRs). The ability to recognize an almost endless variety of antigens is due to this diversity.