30 April 2012
Evolution of the Immune System
Animals are constantly bombarded by an immensely varied array of disease causing pathogens including bacteria, fungi, viruses and other parasites. The number of microbes living in the human body outnumber the actual human cells by a factor of 10, and for every single species of animal and plant on Earth, there are viruses that infect them. With the unrelenting threat of disease-causing pathogens all around us, and even within us, how can the constantly vulnerable organisms defend themselves? Evolution has provided an answer to this problem—the immune system. The immune system is a vastly complex orchestra of cells working together to help eliminate potentially harmful pathogens from the body. Some form of host defense is found in every multicellular organism, however there are myriad variations in the immune systems of different organisms. Vertebrates have evolved an acquired immune response, in which a specific immune system is activated to clear an infection that is initially controlled by a non-specific (innate) immune response. This highly adaptable system is important to the survival of vertebrate species. Surprisingly, however, 90% of animals (invertebrates) do not have this kind of response. Despite lacking a seemingly critical adaptation, invertebrates continue to survive and reproduce. Why does it appear necessary for vertebrates to have an acquired response in order to survive, but the more numerous invertebrate species do not? Research indicates that there is an evolutionary lineage of the immune system that stems from the split of invertebrates and vertebrates. Innate immunity, which is found in all animals, is assumed to be at the beginning of this evolutionary tree. After the diversification of species (vertebrates branching from invertebrates), mechanisms of immunity also diverged. In this paper I will first discuss the function of the innate immune system because of its older evolutionary history, followed by the adaptive immune response that evolved later in vertebrate lineages. I will then conclude by placing the development of the innate and adaptive immune system in an evolutionary context. Innate immunity is the first line of defense for an organism and is made up of elements that protect the organism from pathogens. Anatomical aspects such as skin act as an impermeable barrier to infectious pathogens. Chemical and biological factors, including anti-microbial peptides like defensins, are also used to inhibit bacterial growth and prevent colonization. Another immunological factor of the innate immune system are phagocytic cells (macrophages), which are cells that engulf and eliminate foreign pathogens. These cells operate using a variety of different and generalized receptors that recognize a broad range of molecular patterns expressed by pathogens that initiate phagocytosis. One such family of receptors, known as Toll-like Receptors, recognizes common pathogen elements such as bacterial wall components or viral DNA sequences. This component is found in virtually every multicellular organism, ranging from sponges to humans (Muller and Muller 2003). Plants also express proteins that are very similar to toll-like receptors, indicating that this aspect of the innate immune system predates the divergence of plants and animals. The innate immune system is so valuable to an organism’s survival because it is always present and in many instances can prevent pathogen entry or replication. This, in turn, prevents a harmful infection from ever occurring inside the host. Although invertebrates do not have the acquired immune response, recent research has shown that their innate response is more complex than previously thought. Insects can activate their immune systems to remain in a higher state in order to prepare for a pathogen invasion. During bedbug mating, females are frequently injured in the process because males...