A blood test can determine whether you have a healthy amount of white blood cells and immunoglobulins, or antibodies, which help fight infection. But there is no reliable test to determine the overall strength of your immune system. However, cancer patients, especially those diagnosed with blood cancers, such as leukemias or lymphomas, or who are on certain medications, such as chemotherapy drugs or hormone therapy, may have compromised immune systems and should take steps to avoid exposure to infections.