Compare and contrast the BCR and TCR in terms of the following characteristics : (i) Specificity for antigen (ii) Cellular expression (iii) Types of Ag recognized.

SouravSeptember 4, 2024

Answer

Here’s a comparison between the B-cell receptor (BCR) and the T-cell receptor (TCR) based on specificity for antigen, cellular expression, and types of antigens recognized:

(i) Specificity for Antigen

• BCR (B-cell Receptor):
  • Specificity: BCRs are highly specific for particular antigens. They recognize and bind to free, soluble antigens, such as proteins, carbohydrates, and lipids.
  • Mechanism: The BCR binds to the antigen directly through its membrane-bound immunoglobulin (Ig) molecules. Each B cell has a unique BCR that can specifically recognize one particular antigen.
• TCR (T-cell Receptor):
  • Specificity: TCRs are also highly specific but recognize antigens that are presented by other cells. TCRs do not bind to free antigens but instead recognize peptide fragments of antigens that are presented by Major Histocompatibility Complex (MHC) molecules on the surface of antigen-presenting cells.
  • Mechanism: The TCR binds to the antigenic peptide-MHC complex on the surface of other cells. Each T cell has a unique TCR that recognizes specific peptide-MHC combinations.

(ii) Cellular Expression

• BCR:
  • Cellular Expression: BCRs are expressed on the surface of B lymphocytes (B cells). Each B cell expresses a unique BCR that is specific to a particular antigen.
  • Location: BCRs are found on the surface of naive and memory B cells, as well as on plasma cells after activation.
• TCR:
  • Cellular Expression: TCRs are expressed on the surface of T lymphocytes (T cells). Each T cell expresses a unique TCR that is specific to a particular peptide-MHC complex.
  • Location: TCRs are found on the surface of various subsets of T cells, including helper T cells (CD4+), cytotoxic T cells (CD8+), and regulatory T cells.

(iii) Types of Antigens Recognized

• BCR:
  • Types of Antigens: BCRs recognize a wide variety of antigens including proteins, polysaccharides, lipids, and nucleic acids in their native, unprocessed forms. BCRs can recognize whole pathogens, soluble antigens, and macromolecules.
  • Recognition: BCRs can bind directly to these antigens without requiring processing or presentation by other cells.
• TCR:
  • Types of Antigens: TCRs recognize peptide fragments derived from protein antigens that are processed and presented by MHC molecules on the surface of antigen-presenting cells. TCRs do not recognize antigens in their native form but as peptide-MHC complexes.
  • Recognition: TCRs require the antigen to be presented by MHC molecules, and they do not recognize free-floating or unprocessed antigens.