Final answer:
The T cell is crucial for clonal expansion in the immune system as it undergoes clonal selection, getting activated only if it recognizes a specific antigen, which initiates clonal expansion leading to a targeted immune response.
Step-by-step explanation:
The cell type within the immune system upon which all other cell types depend for the signal that leads to clonal expansion is the T cell. Specifically, the process begins with naïve T cells which, upon recognizing processed foreign antigens associated with self-MHC molecules, become activated. This activation initiates clonal expansion, a rapid division by mitosis, producing large numbers of T cells with receptors specific to the activating antigen. In parallel, B cells undergo a similar process of clonal selection and expansion. They get selected and proliferate if they have receptors that match the antigen. The result is a considerable number of lymphocytes ready to combat the pathogen effectively.
Clonal selection is pivotal as it ensures that only T cells with receptors specific to an antigen are selected. This specificity is vital for an effective adaptive immune response. These selected T cells and B cells then give rise to clones that retain the specificity of the original cell, either to directly attack the pathogens or to secrete antibodies against them.