What is the Difference Between BCR and TCR

The main difference between BCR and TCR is that BCR, or B cell receptor, binds directly to the antigen surface, whereas TCR, or T cell receptor, recognizes antigenic peptides with MHC molecules.

BCR and TCR are two receptors that occur in lymphocytes. They are responsible for specific antigen recognition in adaptive immunity.  

Key Areas Covered

1. What is BCR (B Cell Receptor)
– Definition, Structure, Function
2. What is TCR (T Cell Receptor)
– Definition, Structure, Function
3. Similarities Between BCR and TCR
– Outline of Common Features
4. Difference Between BCR and TCR
– Comparison of Key Differences

Key Terms 

BCR, B Cell Receptor, TCR, T Cell Receptor

What is BCR

BCR (B cell receptor) is a receptor molecule that occurs on the surface of B lymphocytes. T helper cells stimulate B cells to proliferate, producing specific antibodies against a pathogen. Also, a clone of B cells produces only one type of antibody. A typical B cell contains around 105 antibodies. Additionally, the initial antibodies produced by the B cells are not secreted to the circulation. However, they are inserted into the cell membrane to serve as BCRs. Meanwhile, the antibodies that are not secreted are called immunoglobulins. Therefore, BCRs are such immunoglobulins on the surface of the B cells.

Figure 1: BCR

Also, binding a specific antigen causes the activation of the B cell receptor. It initiates a cascade of intracellular signaling, which allows the internalization of the antigen-bound BCR for further processing and presenting the antigen to the T cells.

What is TCR

TCR (T cell receptor) is another type of receptor molecule that occurs on the surface of T cells. Its function is to recognize specific antigens bound to the MHC complex. The binding of the TCR into the antigen is degenerated. Therefore, many types of TCR can bind to the same antigen. Structure-wise, TCR is a heterodimer containing two different protein chains. Around 95% of the TCR contains alpha and beta chains, while 5% of the TCR contains gamma and delta chains. Disease conditions such as leukemia can change the ratio.

Figure 2: TCR

Furthermore, when TCR recognizes a specific antigen with an MHC complex, it activates the T cell through signal transduction mediated by specific enzymes, co-receptors, and adaptor molecules to activate and release transcription factors. 

Similarities Between BCR and TCR

• BCR and TCR are two types of receptors that occur in lymphocytes.
• They are important for the recognition of antigens in adaptive immunity.
• They are protein molecules that occur on the surface of lymphocytes.
• They are specific to a particular antigen.

Difference Between BCR and TCR

Definition

BCR (B cell receptor) refers to a trans-membrane protein on the surface of a B cell, while TCR (T cell receptor) refers to a group of proteins found on T cells.

Occurrence

BCR occurs on the surface of B cells, while TCR occurs on the surface of T cells. 

Function

BCR binds to the specific antigens, while TCR recognizes antigens presented by MHC molecules. 

Type of Antigens

BCR recognizes extracellular antigens, while TCR recognizes antigens on MHC molecules.

Binding Site

BCR contains a bivalent binding site, while TCR contains a monovalent binding site. 

Secretion

BCR can be secreted, while TCR cannot be secreted.

Somatic Mutations

The variable region of the BCR undergoes somatic mutations, while the variable region of the TCR cannot undergo somatic mutations. 

Isotope Switch

BCR can undergo isotope switching, while TCR cannot undergo isotope switching. 

Conclusion

In brief, BCR and TCR are two types of receptor molecules that occur on the surface of lymphocytes. BCR occurs on the surface of B cells, and its function is to bind specific antigens. It recognizes extracellular antigens. It has a bivalent binding site. Also, it can be secreted, and its variable region can undergo somatic mutations. Additionally, it undergoes isotope switching. In comparison, TCR occurs on the surface of T cells. Its function is to recognize antigens on MHC molecules. However, it contains a monovalent binding site whose variable region does not undergo somatic mutations. Besides, TCR cannot be secreted as well. Therefore, the main difference between BCR and TCR is their function.

References:

1. Minervina, A., Pogorelyy, M. and Mamedov, I. (2019), T-cell receptor and B-cell receptor repertoire profiling in adaptive immunity. Transpl Int, 32: 1111-1123. 

Image Courtesy:

1. “Bcellreceptor” By Clarkdn – Own work (CC0) via Commons Wikimedia
2. “Antigen presentation” By user:Sjef – Own Work (CC-BY SA 3.0) via Commons Wikimedia