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T cells (T lymphocytes) derive their names from the organs in which they develop in the thymus. They arise in the bone marrow but migrate to the thymus gland to mature.The diverse responses of T cells are collectively called cell-mediated immune reactions. This is to distinguish them from antibody responses, which, of course, also depend on cells (B cells). T cells cannot recognize antigen alone, as for T cell receptors (TCRs), they can recognize only antigen bound to cell-membrane proteins (MHC molecules). TCRs have different structures thus they bind to different molecular structures and have different genetic codes. Like antibody responses, T cell responses are exquisitely antigen-specific, and they are at least as important as antibodies in defending vertebrates against infection. Indeed, most adaptive immune responses, including antibody responses, require helper T cells for their initiation. Most importantly, unlike B cells, T cells can help eliminate pathogens that reside inside host cells.