The Altruistic Immune System (Draft)

Immunity is a resource investment that benefits not only the host organism but also others in its community by preventing the transmission of infectious pathogens. Immunity presumably presents an energy cost to the host for system maintenance as wells as the risk of detrimental effects to the host organism during appropriate or inappropriate immune response. An individual organism living in a community faces a choice as to the quantity of resources (“QR”) to invest in its immune system. A self-interested individual may choose the QR necessary to optimize its own survival. However, if all or most individuals invest a super-optimal QR such that a primary infected individual more effectively fights infection, transmission rates may fall and thereby benefit the community as a whole. This raises the possibility that cooperative population dynamics (ref) may cause QR to evolve to a super-optimal level. As shown in (ref), unicellular organisms living in a spatially structured environment will in some cases cooperatively use a slow rate/high yield respiratory path for ATP production that confers community benefit although each individual organism has the option of exploiting its neighbors by shifting to a high rate/low yield pathway.

In humans, a large portion of the population is affected by allergies, asthma, rheumatoid arthritis and other conditions that may be caused by a ‘hyper-vigilant’ immune system. These conditions may be thought of as part of the cost or burden of the human immune system.

From an individual host's viewpoint, if QR is too low, infection is likely to kill the host but if QR is too high, resource depletion will likely kill the host. So the optimal QR for host survival lies between the two extremes and is influenced by the mix and virulence of pathogens in its environment as well as the probability of primary and secondary infection. In a community of interacting individuals, as QR rises, the probability of secondary infection decreases thus pressuring QR to fall. Similarly, as QR falls, the probability of secondary infection increases thus pressuring QR to rise. However if QR is uniformly high in a community, the decrease in individual survival from the higher QR may be more than offset by an increase in survival from lower secondary infection and thus lead to a net benefit to the community. This raises the following interesting questions: will QR in a community of evolving individuals tend toward the individually-optimized QR or some higher value based on an "altruism" effect? Will individuals increase their QR beyond that required to optimize their own survival so as to benefit other individuals and thereby the community? And will they do so in the face of the evolutionary situation embodied in game theory's "prisoner's dilemma", i.e., if all members of a community invest a high QR, an individual can "defect" by decreasing its own QR and thus enhance its own survival probability at the expense of the community.

In order to explore these ideas, I have created a simulation. Results will be discussed in the next iteration of this post.