Publisher Summary The pulmonary immune system is constantly exposed to pathogens within the inhaled air; therefore, it becomes essential for the system to determine whether an antigenic molecule represents a hazard or not. Under normal conditions, most infectious agents or foreign antigenic materials do not signal to the host and are processed without requiring an inflammatory response. It is only when infectious burden becomes dangerous that the pulmonary accessory cells process and expose the antigen to T lymphocytes so that the required action is taken. This chapter reviews current concepts on the recruitment, homing, and activity of lymphocytes in the lower respiratory tract. The chapter highlights the activity pattern shown by lung T cells in response to immunological challenges in addition to explaining relevant abnormalities detected in B, T, and natural killer cells in asthma and chronic obstructive pulmonary disease (COPD). Antigen-specific lung lymphocytes have evolved a number of effector mechanisms to respond to foreign antigens, ranging from direct cytotoxicity mechanisms to the secretion of lymphokines that have the ability to activate themselves or other pulmonary immunocompetent cells.