Final answer:
CCL19 and CCL21 attract T cells to lymphoid tissues, assisting their activation through antigen recognition. These chemokines play a critical role in the immune system's response, including clonal expansion and the function of different subsets of T cells.
Step-by-step explanation:
True, CCL19 and CCL21 are chemokines involved in the immune system's regulation. These molecules are known for their role in attracting T cells to sites within lymphoid tissues, where the T cells can encounter antigen-presenting cells and become activated. Clonal selection and expansion, as depicted in Figure 21.19, highlight the process by which T cells with specific receptors (clones) are selected and expanded when they recognize antigens on a pathogen. This process is vital for an effective immune response, including the production of cytokines by helper T cells and the destruction of infected cells by cytotoxic T cells.
Regarding the details provided in the visual connection questions, Helper T cells are indeed CD4+ and release cytokines, while cytotoxic T cells are CD8+ and have the role of killing infected cells. The statement that MHC II is found on most body cells is false; instead, MHCI molecules are present on all nucleated cells, and MHC II molecules are mainly expressed on professional antigen-presenting cells.