The immune system evolved specifically to repel and eliminate pathogenic microorganisms such as bacteria, viruses and fungi, which can invade the body and cause infections. The cells of the immune system are programmed and trained to recognize pathogenic microorganisms and produce immune mediators that will kill the pathogen, prevent the pathogen from invading tissues and cells, and stop the pathogen from replicating and transmitting to nearby non-infected individuals. The immune system employs two waves of host-defense – an early wave (referred to as the innate phase of the immune response) that helps to limit infection and a late wave (referred to as the adaptive phase of the immune response) that clears the pathogen and repairs tissues and cells that were damaged during the infection. The cells that direct the adaptive immune response are special in that these cells are long-lived and are programmed to remember the encounter with the pathogen.

Vaccines to pathogens, like influenza or SARS-CoV-2, are designed to elicit these “memory” immune cells that will remain in the body and provide rapid protection when the pathogen is encountered again. The cells of the immune system, including the memory cells, are present in the blood and lymphoid tissues (e.g. lymph nodes) where the cells can circulate throughout the body looking for invading pathogens. Immune cells also reside in tissues like the lung and the skin and the intestine where they can serve as sentinels and barriers to pathogens that are transmitted through the air (e.g. tuberculosis), through food (e.g. Listeria) or through breeches in the skin (e.g. Zika virus). Individuals suffering from immunodeficiency syndromes caused by genetic mutations that affect the development or function of immune cells involved in host-defense are predisposed to infections.

Immunologists who study immune responses to infection address the fundamental questions of how the immune system recognizes and then eliminates pathogens from the body. Immunologists also conduct studies to determine the programming signals that are required to generate and maintain immune memory with the goal of using this knowledge to design and test vaccines that will protect individuals from developing life-threatening infections.

Learn more about UAB research focused on host-defense and vaccination here.