Final answer:
The cell type within the immune system that initiates clonal expansion is the T cell. T cells undergo clonal selection and expansion upon encountering specific antigens, leading to a substantial increase in the number of lymphocytes ready to combat the infection.
Step-by-step explanation:
Cell Type Essential for Clonal Expansion in the Immune System
The cell type within the immune system on which all other cell types depend for the signal that leads to clonal expansion is the T cell. Clonal expansion occurs when specific T cells, which have receptors matching a specific antigen, are selected through the process of clonal selection. These selected T cells then undergo rapid mitosis to produce many identical copies, thereby amplifying the immune response. In this context, there are two main types of T cells: cytotoxic T lymphocytes (Tc) that directly kill infected cells and helper T cells that regulate the immune response. This clonal selection and expansion ensure that the body has sufficient numbers of specific lymphocytes to effectively fight an infection.
Both B cells and T cells undergo clonal selection and expansion. In the case of B cells, those with receptors specific for an antigen are selected and expanded to form plasma cells that secrete antibodies. Whereas for T cells, antigen specificity is established by the unique amino acid sequences and three-dimensional shapes of the antigen-binding sites on T cell receptors. Naive T cells mature and become activated upon encountering processed foreign antigens in combination with self-MHC molecules.