Abstract Naïve CD8+ T cells are activated through interactions with antigen (Ag) bearing dendritic cells (DCs) in lymph nodes (LNs). Multiphoton intravital microscopy (MP-IVM) of LNs has demonstrated that T cell activation occurs in three phases. The duration of the initial phase of transient, serial DC-T cell interactions is inversely related to Ag dose. It had been assumed that the second phase, characterized by stable DC-T cell contacts, is necessary for T cell activation, however, here we show that this is not always the case. T cells interacting with DCs presenting Ag at low abundance and with a short half-life did not transition from phase one to phase two, whereas a higher dose of the same Ag yielded robust phase two transition. T cells exposed to either antigenic constellation proliferated and developed equivalent effector functions, including cytokine production and cytotoxicity, over the first two days after priming. There are profound differences in the transcriptome at 12h and 24h after activation between CD8 T cells exposed to antigenic constellations that did or did not promote phase two interactions. Only T cells that had undergone stable contacts gave rise to memory, whereas activating conditions that did not support stable contacts yielded immunological amnesia. T cells can make fate decisions in vivo within a few hours after Ag exposure that result in long-term memory or abortive effector responses and correlate with the kinetics of T cell-DCs interactions.