Phytohemagglutinin (PHA) is a plant lectin commonly used to stimulate and test responses of the immune system and is known to induce T cell proliferation, agglutinate human leukocytes, and yield adjustments in lymphocyte populations. What is not well know is how responses to PHA correlate with a host's ability to resist or recover from pathogen invasion. This study uses information from previously published studies to determine whether or not PHA can be a good indicator of disease severity or disease resistance in a host. With PHA having the abilities that it does, immune responses to PHA may correlate with responses important for pathogen resistance and clearance. Such a relationship could only be uncovered if in vivo or in vitro responses to PHA are measured and, independent from the PHA challenge, symptoms and/or mortality rates of hosts are documented after pathogen exposure. An in vitro response can be detected by measuring cellular proliferation in response to PHA followed by separate cell cultures exposed to a pathogen. While an in vivo response can be detected by measuring variation in swelling in response to an injection of PHA. In reviewing a broad range of articles that meet my criteria, the majority of articles failed to show a strong relationship between PHA and disease severity or disease resistance. Therefore, immunologists must consider the usefulness of the PHA tests as a measure of immunocompetence, which is a host's ability to predict response to a pathogen. According to the literature, using PHA does not predict responses to pathogen invasion. However, it is possible that with carefully designed experiments, it could be determined that PHA does provide an indication of pathogen resistance in certain host species exposed to specific pathogen.