Respuesta :
Immune receptors can recognize a nearly infinite number of molecules. Receptor diversity is generated through genetic recombination. The BCR and TCR loci contain a large number of small genes referred to as variable (V), diversity (D), and joining (J) genes. BCR heavy chains are encoded by different combinations of V, D, and J genes, and BCR light chain loci have only V and J genes. TCRs are similar in that one chain (α) has only V, and J genes and the other (β) has V, D, and J genes.
From a diversity perspective, BCRs and TCRs are very similar with a few significant exceptions, BCRs have increased functional diversity through their constant region (C-region) genes. There are five classes of C-regions (α, δ, ε, γ, μ), and a BCR is defined by its constant region (IgA, IgD, IgE, IgG, and IgM). The C-region determines what happens after the antibody binds an antigen. As an immune response evolves, a BCR can change its C-region (class switching) to alter its functional role.
C-regions and class switching lead to another important difference between BCRs and TCRs. TCRs are always bound to cells whereas BCRs can be either membrane-bound or free in solution. The BCR constant region plus alternative splicing of domains within the C-region determine a BCR’s membrane-bound or free state.
Finally, BCRs can be “tuned” in that their ability to bind an antigen can improve over time through a process called somatic hypermutation (SHM).
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