Final answer:
The T cell receptor recognizes antigen fragments presented on the surface of cells by major histocompatibility complex (MHC) proteins. There are two main types of MHC proteins: MHC I, which presents to cytotoxic T cells, and MHC II, which presents to helper T cells. TCRs and BCRs are key components of the immune system's ability to identify and respond to antigens.
Step-by-step explanation:
The T cell receptor recognizes antigen fragments housed in cell membrane proteins called "major histocompatibility complex" (MHC) proteins.
T cell receptors (TCR) are molecules on T cells involved in the recognition of processed foreign epitopes. These foreign antigens are presented along with MHC molecules which are encoded by a cluster of genes. There are two classes of these molecules, MHC I and MHC II, and they present antigens to different types of T cells. MHC I presents to cytotoxic T cells, while MHC II presents to helper T cells. The nature of the MHC molecule dictates the type of T cell that will be activated in the immune response.
TCRs recognize a complex of antigen fragment and MHC molecule on the surface of antigen-presenting cells. The process is termed as antigen processing and presentation. Just like B-cell receptors (BCRs), which are membrane-bound immunoglobulins (IgD and IgM) that serve as the antigen receptors on the B cell surface, TCRs are crucial for specific immune responses. BCRs bind directly to epitopes of free antigens, whereas TCRs require antigens to be processed and presented by MHC molecules on antigen-presenting cells.