In order for us to properly
understand the diseases that make us sick we must examine the entire mechanism
the pathogen (disease causing agent) uses to invade our bodies. Much
experimental work has been done to understand the pathogen mechanism once it
has generated an immune response in the body, however one of the most important
steps in the mechanism, which hasn’t really been experimentally studied, is the
mode of entry. There are two major ways that a pathogen can enter the body,
either through ingestion (orally) or systematically (a prick or needle). When a
pathogen enters the body systematically it bypasses much of the innate
physiological and anatomical barriers that prevent pathogen entry, like our
skin, sweat, oils and mucus we secrete. In contrast, a pathogen must overcome
many more barriers when entering the body orally, for instance saliva.
It has been shown in a recent
experiment that the path of pathogen entry affects the rate at which the pathogen
is eliminated and the way that it responds to the same type of pathogen. In the
experiment equal amounts of both male and female Drosophila melanogaster flies were administered with one of two
treatments of P. entomophila bacterial
strain. Half the male and female flies were introduced to P. entomophila orally (BactOral), whereby food plates were coated
in the bacterial strain. The other half of male and female flies was injected
in the thoracic (area where the legs and wings attach) region with P. entomophilia (BactSys). Controls for
both the injection (ContSys) and food treatments (ContOral) were also observed
in the experiment. The mortality of the flies was measured for at least 10
days. It was found that the survival rate of the flies was much higher when the
bacterial strain was introduced orally. The evolution of resistance to the
particular bacterial strain also developed much faster when P. entomophilia was introduced orally.
These results reaffirm the theoretical
notion that oral pathogens have to overcome more physical barriers in order for
them to enter the body cavity. This is in contrast to a systematic infection
because the physical barriers, like the skin have already been bypassed and the
bacteria have been directly injected into the body cavity. So when a pathogen
enters systematically the body must rely on other, more active ways of
identifying and eliminating the invader, namely through members of the immune
system that belong to the adaptive (more-specific) response like plasmatocytes.
These more complex and specific ways of eliminating the invader are slower to
act because B cells must be activated to produce antibodies, that than must
find the pathogen and kill it, whereas much of the pathogen that was introduced
to the mice orally didn’t really require the activation of the adaptive immune
response. It only had to rely on the innate (more general) immune response and
physiologic barriers that were already in place like the mucosa layers of the
respiratory and gastrointestinal tracts to prevent pathogen entry into the body
cavity. Since these defenses are already localized to the site of pathogen
entry and need no further activation, the immune response is much faster when
the pathogen is administered orally rather than systematically.
