Molecular biology and immunology for clinicians 24: phylogeny of the immune system

The human immune system consists of layer upon layer of response, communication, and coordination mechanisms added during its evolution throughout the last half billion years. Given the parsimony of nature, it is not surprising that many of the systems prominent within the human immune response are recognizable within the immune systems of less advanced species. Insights drawn from these species and common sense suggest that "autoimmune" reactivity was not the original or primary reason for the evolution of these mechanisms. Also, it becomes clear that certain functions thought to be "immune" may actually be nonspecific, really innate functions served by these ancient mechanisms. There is good evidence of communication between levels of the immune system, the innate and adaptive systems working with each other, often the former (not antigen-specific) preceding and serving to trigger or magnify the latter (antigen-specific). A future article in this series will focus more on the innate system and how it interdigitates with the adaptive system.