Purpose of review: This review will contest the idea that the adaptive immune system of mammals represents an evolutionary advance which superseded the less evolved, innate immune system of invertebrates. General conclusions about the evolution of immune systems will be applied to current infectious diseases of humans common in conditions of poverty.
Recent findings: Many invertebrates have mechanisms for diversifying the repertoire of receptors specific for pathogen-associated molecules. The lamprey uses true genetic rearrangement to produce a repertoire comparable to that of our antibody-based system. Both adaptive immune systems may have evolved independently from an efficient, pre-existing, lymphocyte-based, innate immune system, the importance of which has only recently been described in mammals.
Genetic variation in human populations is associated with susceptibility and resistance to infectious diseases. Specifically, human leucocyte antigen and natural killer receptor alleles are associated with susceptibility or resistance to HIV. However, the extent to which these will exert long-term selective pressure is unclear.
Summary: Human pathogens do to exert selective pressures on the immune system, but these are unlikely to do more than change allele frequencies in the short term. In the long term, pathogen-driven evolution seems to promote expansion of recognition repertoire in adaptive and innate immune systems.