This review has examined the possible role of CMI in providing protection against three pathogens that can be opportunists in the lung. Monoclonal antibodies that identify the cellular components of the immune response and recombinant cytokines are important tools to better understand how pulmonary immunity is regulated. Although not discussed in detail, recombinant microbial antigens are useful for understanding various aspects of protective immunity and immunosuppression as well as for advancing vaccine development. There are important problems to address in order to continue steady progress in understanding pulmonary defenses, including some of those mentioned in this brief review. There should be an increased use of infectious models that more closely mimic naturally occurring infections, and comparisons should be made between results obtained with parenteral versus intrapulmonary routes of infection.