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Before we begin an examination of the immune system it is worthwhile to consider whether an animal is even susceptible to a disease. While not really part of the immune system, this can still determine the outcome of an encounter of an animal with a pathogen.
Whether a pathogen can cause disease in a host is dependent not only on the virulence of the pathogen, but also on the genetic background and health of the host. Some species have an innate susceptibility to a pathogen not shared with other related hosts. For example, humans are the only host for the agents of syphilis, gonorrhea, measles and poliomyelitis. In contrast, we have innate resistance to canine distemper virus and feline leukemia virus. These differences in susceptibility may be related to a number of factors. The resistant host may lack a cellular receptor required by the pathogen for attachment or penetration of the host. The temperature of the host may also preclude the growth of a potential pathogen. For example, Mycobacterium tuberculosis does not cause illness in frogs since it cannot grow well at temperatures much below 37 °C. Being cold-blooded, frogs do not normally reach these temperatures. Pathogens may also require a nutrient that is not available in a resistant host. Purine-requiring strains of Salmonella typhi cannot cause disease in rats, since rats do not make purines available for the pathogen to grow on. A final possibility is the lack of a target site for a toxin that the microbe produces. Rats injected with diphtheria toxin show no ill effects because the rats do not contain a receptor on their cell surfaces that allows the toxin to enter the cells. Since the toxin cannot enter the cells, it cannot have its toxic effect.
Individuals within a species can also exhibit different susceptibility or resistance to a pathogen when compared to others. The age of an individual can have an overall effect on disease resistance, with the very young and the very old being more susceptible to infection by a wide variety of pathogens. Stress in the form of extreme exertion, shock, a change in environment, climate change, nervousness or muscle fatigue can have a negative impact on health. Each of these conditions is thought to increase the release of cortisol from the adrenal cortex, causing a suppression of the inflammatory response, thereby facilitating infection.
A chronic disease or the treatment of that disease can also weaken the body of an individual and open it up to secondary acute infections. The normal immune defenses can be impaired by serious underlying illnesses, such as AIDS, Hodgkin's disease and diabetes. Treatment of many cancers involves killing fast-growing cancer cells. This has the unwanted side effect of killing fast-growing non-cancerous cells, including the ones that make up the immune system. Patients undergoing chemotherapy and radiation therapy are therefore more susceptible to infections. The increasing numbers of transplant patients, who must take immunosuppressive drugs to prevent rejection, are also at higher risk for infection than the general population.
Poor diet is another factor that can suppress the immune system. A number of studies link vitamin and protein deficiencies to a higher rate of infection and general immune suppression. Malnutrition is in part responsible for higher rates of infection and higher infant mortality in developing nations. Being obese is also detrimental to the overall health of an individual. Morbidly obese people are more susceptible to invasive streptococcal infections. Obesity also leads to type 2 diabetes, which in turn makes people more susceptible to infectious diseases. As you can see, eating too much of the wrong kinds of foods and becoming overweight can lead to all kinds of trouble.
There are also sexual differences in the degree of susceptibility to disease. In some cases, anatomical differences cause members of one sex to be more resistant to infection than those of the opposite sex. Obviously, men cannot suffer infections of the uterus and women do not develop prostatitis. More subtly, urinary tract infections are 14 times more likely in women, because bacteria more easily travel up the short 4 cm female urethra than the longer 18 cm male urethra to reach the bladder. In addition, because of the anatomical closeness between the opening of female urethra and anus, intestinal bacteria easily gain entry into their urinary tract.
Differences in genetic background between individuals can also have an influence on susceptibility. For example, Eskimos, Native Americans and Asians are more susceptible to tuberculosis than are Caucasians. Also, individuals that are heterozygous for the gene that causes sickle-cell anemia are more resistant to the malaria protozoan. Another example comes from the AIDS epidemic. There are rare cases of individuals who have had frequent unprotected sexual encounters with virus-carrying partners yet have not contracted the disease. It turns out these individuals have a mutation in a cellular surface protein that makes it impossible for the AIDS virus to enter their cells. Without the presence of the normal surface protein, the virus cannot infect and cause disease.
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