The decrease of CD4+/CD8+ ratio and IFN-γ production capacity, together with granulocytopenia represented immunosenescence, and the decrease of complements and antimicrobial peptides represented the declined immune function, all these performances indicated that the defense ability of thyrotoxicosis mice declined seriously, which were the main causes of susceptibility to infection in the elderly. The gene discussed is IFNG; the disease is thyrotoxicosis.