Methods of sorting T lymphocytes in a microfluidic device are provided. The methods can include flowing a fluid sample comprising T lymphocytes through a region of a microfluidic device that contains an array of posts. The array of posts can be configured to have a critical size (Dc) that separates activated T lymphocytes from naïve T lymphocytes. Also provided are microfluidic devices having an array of posts configured to separate activated T lymphocytes from naïve T lymphocytes, compositions enriched for T lymphocytes, particularly activated T lymphocytes that are known to be reactive to an antigen of interest, and methods of treating subjects suffering from a pathogenic disorder or cancer by administering such compositions.