Evolution of the Immune System

The immune system is continuously evolving as new pathogens, viruses and other causes for infections and disease continue to evolve. The immune system is an organisms defense against attacks from pathogenic organisms and toxic compounds that affect an organisms well-being. All organisms have some form of immune system as all forms of life need protection from competitors, parasites, infections and other harmful factors in order to increase survival and their time available for reproduction.

Charles Darwin’s theory of evolution has an overwhelming amount of evidence, in all fields of biology, that supports the theory. The mechanism of evolution is based around the concept of natural selection whereby organisms with traits and genes, more beneficial for their environment, are more likely to reproduce and pass on their traits and genes to more offspring. The evolution of the immune system has followed the same process as individuals with more effective immune systems are likely to survive and pass on their immunity to their offspring.

An organisms environment is shaped by abiotic and biotic factors such as temperature, precipitation, wind, predation, competition and disease. As environmental factors change, the level of benefit gained from certain genes varies and evidently the individuals with the most successful immune systems in their given environment are more likely to produce offspring and have their traits expressed in future generations. A process following the mechanism of natural selection and evolutionary theory.

The natural selection of fitter genes leads to the eventual elimination of undesirable genes and causes morphological, physiological and behavioral changes to a species over a number of generations. The evolution of the immune system can be described as having followed normal Darwinian processes as resistance to disease and infection directly affects an individual’s fitness.

Innate Immune System

The immune system of all higher vertebrates can be divided into two branches: the innate immune system and the adaptive immune system. The innate immune system acts quickly on first signs of an infection and has a general approach to attack all variations of infections. This is the oldest form of immunity and can be found in all organisms from mammals to bacteria.

Compounds involved in the innate immune system include cytokines, antibiotic peptides, enzyme cascades and complements; each of which working in different ways to help fight off infections and often stemming from different ancestral molecules. Antibiotic peptides, for example, are believed to have evolved from proteins with positive charges such as DNA and RNA-binding proteins. This theory is based on evidence that particular toad and catfish peptides have derived from histones. Mammals and other higher vertebrate families have immune systems which have developed a more specific adaptive immune system, although there is still a great reliance on the innate immune system as it acts far more rapidly on infections.

Adaptive Immune System

The adaptive immune system is slower to react but far more specific in its approach to fighting antigens. This part of the immune system develops antibodies specific to certain antigens and provides a very efficient response to a repeat attack from the same pathogen via the production of antibodies developed in the previous infection.

The adaptive immune system is found in all jawed vertebrates and is believed to have evolved from genetic modifications of components of the innate immune system. This genetic mutation has allowed for the development of antibodies, T cells, B cells and the major histocompatibility antigens (MHAs) which gave individuals a survival advantage due to an increased ability to defend against potentially lethal pathogens. Evidence suggests that two genes, known as RAG1 and RAG2 (essential for VDJ recombination in B-cells), had evolved from a genetic element known as a ‘transposon’ which are able to cut themselves out of a DNA sequences and place themselves into another.

The adaptive immune system can be considered as a more efficient system and therefore requires less energy to fight off similar infections. The energy saved can be used in other areas such as growth, hunting and reproduction and is therefore a beneficial gene, producing positive feedback in the process of natural selection.

It is important to consider the notion that phyla such as Echinodermata (star fish) and Annelida (worms) have immune systems not necessarily less evolved than our own but have rather followed a different evolutionary path. The particular species that belong to these phyla are not our ancestors but have evolved along-side of us and, more correctly, are our cousins whom share common ancestors.

As environments change and new diseases and problems arise, the immune system will continue to adapt and evolve to the new challenges it faces. Immune systems and pathogens will continue to co-evolve as long as life exists following the mechanism of evolution by natural selection.
[p]Last edited: 24 May 2015[/p]