This paper argues that immunological memory is in the same class of associative memories as Kanerva's Sparse Distributed Memory, Albus's Cerebellar Model Arithmetic Computer, and Marr's Theory of the Cerebellar Cortex. This class of memories derives its associative and robust nature from a sparse sampling of a huge input space by recognition units (B and T cells in the immune system) and a distribution of the memory among many independent units (B and T cells in the memory population in the immune system). Keywords: Immunological Memory, Associative Memory, Cross-Reactive Memory, Original Antigenic Sin, Sparse Distributed Memory. 1 Introduction Cowpox vaccination, used to protect humans from smallpox, was the first known deliberate use of associative recall in the immune response (Jenner, 1798). The modern investigation of associative recall began with the observation that antibodies induced during an influenza infection often have greater affinity to prior strains of influenza than t...