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I think you have an over-eager author at your hand or simply a slip.
What they mean here is lymphocytes of the Th class, they activate the T and B lymphocytes. Other than that the only plausible cell, a helper-activated-by-a-helper would be the γδ T-cell because of their alternate T cell receptor and shared characteristics with helper T-cells.
Sorry if this sounds confusing. Basically, Th-cells are the only "helper cells" with γδ T-cells being the only cells sharing some similarities with helper cells. (Although the CD+4 T-cell can also be considered a helper).
Did that just make any sense at all or am I rambling?