What is the Difference Between Clonal Selection and Clonal Expansion

The main difference between clonal selection and clonal expansion is that clonal selection is the process of matching antigens on antigen-presenting cells with the antigen receptors on B and T cells, whereas clonal expansion is the production of genetically identical daughter cells of activated T and B cells by cell division.

Clonal selection and clonal expansion are two processes that occur during the differentiation of lymphocytes in the bone marrow. 

Key Areas Covered

1. What is Clonal Selection
– Definition, Features, Importance
2. What is Clonal Expansion
– Definition, Features, Importance
3. Similarities Between Clonal Selection and Clonal Expansion
– Outline of Common Features
4. Difference Between Clonal Selection and Clonal Expansion
– Comparison of Key Differences

Key Terms 

Clonal Expansion, Clonal Selection

What is Clonal Selection

Clonal selection is the reaction of the receptors of B lymphocytes with antigens presented in antigen-presenting cells. It occurs in secondary lymphoid organs such as the spleen and lymph nodes. Australian doctor Frank Macfarlane Burnet introduced the concept of clonal selection in 1957. It explains the generation of antibodies with great diversity during the initiation of the immune response. It allows the immune system to respond to infections with T and B lymphocytes, selecting specific antigens. Here, a specific antigen activates a pre-existing group of T and B lymphocytes to produce clones of specific lymphocytes that produce antibodies.

Figure 1: Clonal Selection

Furthermore, the clonal selection theory describes four tenets. They are:

• A single type of receptor occurs in each lymphocyte with a unique specificity.
• The activation of the lymphocytes occurs upon receptor occupation.
• Activated lymphocytes form differentiated effector cells that contain specific receptors. They are identical to the parent cells.
• The lymphocytes that contain receptors specific for self-antigens are destroyed in early development. 

What is Clonal Expansion

Clonal expansion is the rapid proliferation of activated B cells, producing many identical B lymphocytes. It occurs in response to the cytokines produced by T4 helper lymphocytes. During proliferation, B lymphocytes undergo affinity maturation due to somatic hypermutations. Somatic hypermutation is a genetic process that accumulates point mutations in the V region of the antibody. On the other hand, affinity maturation is the production of antibodies with several-fold greater affinity to a specific antigen. Both processes allow B lymphocytes to produce antibodies specific to a particular antigen. B lymphocytes with specific antigen receptors undergo proliferation in clonal expansion.

Figure 2: Clonal Expansion

Moreover, a single activated B lymphocyte proliferates and produces approximately 4000 cells within seven days. The main function of activated B lymphocytes is to produce antibodies. 

Similarities Between Clonal Selection and Clonal Expansion

• Clonal selection and clonal expansion are two processes that occur during the differentiation of lymphocytes.
• They produce lymphocytes specific to the antigens.
• They occur in lymphocytes. 

Difference Between Clonal Selection and Clonal Expansion

Definition

Clonal selection refers to a hypothesis that an individual lymphocyte (specifically, a B cell) expresses receptors specific to the distinct antigen, determined before the antibody ever encounters the antigen. At the same time, clonal expansion refers to the clonal proliferation of cells responsive to a specific antigen as part of an immune response.

Process

Clonal selection is the process of matching antigens on antigen-presenting cells with the antigen receptors on B and T cells. Meanwhile, clonal expansion is the production of genetically identical daughter cells of activated T and B cells by cell division.

Importance

Clonal selection is important for identifying antigens by B lymphocytes, while clonal expansion is important to produce many cells that specifically react to antigens.

Conclusion

In brief, clonal selection and clonal expansion are two processes important for producing antigen-specific lymphocytes. Clonal selection is the matching of antigens with the antigen receptors on lymphocytes. It is important for the identification of antigens. In comparison, clonal expansion is the production of genetically identical B lymphocytes and T lymphocytes that contain specific antigen receptors. It is important in reacting lymphocytes with antigens. Therefore, the main difference between clonal selection and clonal expansion is their process.   

References:

1. (2022a, April 9). 13.1E: Clonal Selection and Clonal Expansion. Biology LibreTexts. 

Image Courtesy:

1. “2222 Clonal Selection of B Cells” By 2222 Clonal Selection of B Cells hu.jpg – Own work (CC-BY 3.0) via Commons Wikimedia
2. “Clonal Expansion of T Cells” By JandS505 – Own Work (CC-BY SA 3.0) via Commons Wikimedia