Clonal expansion:

Mature animals which are immunocompetent have large number of antigen reacting cells of T lymphocytes and B lymphocytes. Antigen specificity of each and every clone is by antigen binding receptors that are present on the lymphocytes clone. At some stage during the maturation period, T lymphocytes and B lymphocytes undergo a random change in their gene arrangements which obviously decide the specificity of each antigen of T and B lymphocytes.

 Function of antigen is complex and crucial when antigen interacts with T and B lymphocytes. It leads to the expansion of antigen specificity and immunocompetent populations or communities. This sort of expansion is referred as clonal expansion. In clonal expansion, each antigen will bind only to the specific T or B cell. After this, they are stimulated to repeatedly divide as clone of cells. Each clone has the same antigenic specificity as that of the parent cells. During Clonal selection, T and B lymphocytes gradually increase.

Clonal expansion framework

Clonal expansion is a type of characteristic of adaptive immunity. It helps in better understanding of recognition of cells, either self or non self in addition to the specificity recognition.

• Specificity is feasible as the antigen receptor binds only to specific antigen thereby resulting in the expansion of cells and are mobilized for immunocompetence.
• Self or non self recognition is due to development of cells, elimination, lymphocytes’ self reactive receptors and/ or adult cell function suppression.
• Immunologic memory is a vital role of Clonal selection. The Clonal selection results in amplification of cells. Some cells may have more life span than normal cells which are referred as memory cells.
• Prediction of specific immune response like graft rejection.

Maturation and Clonal selection of B lymphocytes

When an antigen is encountered, then T lymphocytes develop primary response. This happens in first 5-7 days just before antibody production starts. When a cell is attacked again rapidly, then they develop secondary response. Depending on the intensity and rapid growth of cells, it is easy to make a distinction between primary response and secondary response. The B cells’ clonal propagation produces antigen specific antibody like plasma cells and memory B cells. Antibody production reaches peak in 14 days after which it comes down as plasma cells die. Secondary response refers to the active specificity of memory B cells. Memory cells response is active than normal B cells and are involved in primary response. However, plasma cell production corresponds to secondary response. When T lymphocytes recognizes specific antigen MHC (Major Histo Compatible) complex, domino effect results in clonal proliferation of T cells as helper T cells, memory cells and complementary T cells.

Instance of specific immune response

Transplantation of skin in three mice A, B and C is a best example for specific immune response. When mice C is grafted on mice A, then in the first 10-14 days, the mice develops primary response and the graft is rejected. On repetition, the mice develops vigorous rejection compared to first graft rejection. On the other hand, an engrafted mouse C is grafted with the skin of mice B, primary responses is developed in 10-14 days and is rejected. By repeating the same, it results in graft rejection of skin. When graft rejection increases, clonal cells like helper T cells and complementary T cells are produced. This clonal expansion results in graft rejection.