A new markers for insulin production decline in Type 1 diabetes has been found in the ratio the CD4 naïve (CD45RO-CD62L+) to central memory (CD45RO+CD62L+) and in the level of CD4 central memory T-cell subpopulations. A method of diagnosing autoimmunity and its progressiveness, more specifically diabetes, pre-diabetes, a susceptibility to diabetes mellitus, or the level of effectiveness of therapy/intervention modality for one or more of such conditions in a subject can be conducted by determining level of CD4 naive (CD45RO- CD62L+) T-cells by immunofluorescence analysis of a sample extracted from a subject； determining level of CD4 central memory (CD45RO+CD62L+) T-cells by immunofluorescence analysis of a sample extracted from a subject, and quantitatively relating the levels of the CD4 naive and central memory T-cells, wherein a low ratio of CD4 naive T- cells to CD4 central memory T-cells and/or high CD4 central memory T-cell indicates autoimmunity, a susceptibility to autoimmunity, diabetes, pre-diabetes, a susceptibility to diabetes mellitus or ineffectiveness of a treatment for one or more of such conditions.
