Final answer:
TCRs and antibodies are structurally similar yet functionally different. TCRs are membrane-bound and pivotal in cell-mediated immunity, while antibodies are secreted and essential to humoral immunity. Option 'D' accurately describes their distinctions.
Step-by-step explanation:
TCRs (T cell receptors) and antibodies share structural similarities but have distinct functions and physical forms. TCRs are similar to antibodies in having variable and constant regions, with the variable region providing the antigen-binding site. TCRs, made of two peptide chains (α and β), are membrane-bound structures on T cells. In contrast, antibodies, which are typically Y-shaped and made up of four peptide chains (two heavy and two light chains), are often secreted by B cells after they bind to antigens.
TCRs and antibodies differ fundamentally in their roles within the immune response. TCRs are involved in cell-mediated immunity, recognizing processed antigen peptides when presented by MHC (Major Histocompatibility Complex) molecules on APCs (antigen-presenting cells). Antibodies, on the other hand, are crucial to humoral immunity, directly binding to intact antigens, such as those found on pathogens, without the need for presentation by MHC molecules. Antibodies can recognize a variety of epitopes, including proteins, polysaccharides, and lipopolysaccharides, while TCRs are specific for protein epitopes.
The correct option that addresses the differences and similarities between TCRs and antibodies is 'D' – TCRs and antibodies are different in structure and function; antibodies are involved in humoral immunity, while TCRs are involved in cell-mediated immunity.