Final answer:
Clonal selection of helper T-cells involves the activation of T-cells by antigens on APCs, leading to clonal expansion and the generation of specific T-cell receptors to fight off the infection effectively.
Step-by-step explanation:
Clonal selection of helper T-cells is a process where specific T-cells are activated by antigens presented on the surface of antigen-presenting cells (APCs). Naive CD4+ T-cells engage with MHC II molecules on these APCs, recognizing foreign epitopes. This interaction, combined with additional signals, such as cytokines, leads to the activation and proliferation of the T-cell. The activated helper T-cell then assists in the activation of B cells and CD8+ T cells, the latter becoming cytotoxic T cells which are responsible for killing infected cells.
The specificity of a T cell is based on the unique amino acid sequence and three-dimensional shape of its antigen-binding site. During clonal expansion, these T-cells replicate, producing many copies, each with the specific receptor tailored to the antigen. This ensures that the body has a large number of T-cells ready to fight off the infection effectively. The immune response is thus tailored and amplified, conferring a precise defense against specific pathogens.