Final answer:
The T-cell receptor (TCR) is structurally similar to an immunoglobulin Fab fragment as both contain a variable and a constant region important for antigen-binding, but TCRs are less complex and smaller. Genetic rearrangement provides diversity in antigen specificity for both receptors.
Step-by-step explanation:
The T-cell receptor (TCR) does have structural similarity to an immunoglobulin Fab fragment. Both the TCR and the Fab region of an antibody share a common structure with a variable region and a constant region. The variable region is responsible for antigen-binding specificity. However, the TCR is smaller and less complex than immunoglobulins, consisting of only two peptide chains, while immunoglobulins have four peptide chains and form a Y-shaped structure.
Both TCRs and B-cell receptors (BCRs) generate diversity in antigen binding through genetic rearrangement of V, D, and J gene segments that encode the variable domains of their respective receptors. This rearrangement process creates millions of unique antigen-binding sites. Furthermore, structural and functional differences exist such as TCRs being membrane-bound receptors that engage with processed antigens presented by MHC molecules, whereas immunoglobulins can function both as BCRs when membrane-bound, and as free antibody molecules when secreted by plasma cells.