This is correct, also concentrating the cells presenting antigen and the cells that need antigen to activate/multiply increases the rate at which an immune response occurs. I can't remember the exact numbers but lots of T cells can contact dendritic cells per hour when they are stationary in a Lymph Node, increasing the odds that the correct ones are found; and then those activated T cells can go help other cells out, like B cells, which are also located in high numbers in the lymph nodes. It's essentially all about efficiency.