Special susceptibility to or an increased severity of infections
Children have a special susceptibility to or an increased severity of infections for a number of reasons:
- First exposure. Exposure to an agent for the first time (e.g., to parainfluenza or influenza virus) often produces fever and a rather severe illness. A re-exposure, such as may occur in an older child or an adult, is more likely to produce a mild illness, modified by the serum antibodies from the first infection, primarily IgG, and by the antibodies in respiratory secretions, predominantly IgA. Young children are also less likely to have cross-reacting antibodies from a previous infection with an antigenically related organism.
- Small passages. The smaller passages of children (e.g., the bronchi, the larynx, and the Eustachian tubes) are more easily obstructed by edema or secretions.
- Young cells. There is considerable laboratory evidence that rapid growth rates of cells, such as those in fetal tissue, make cells more susceptible to infection with most viral agents than are adult cells (e.g., Coxsackie B viruses infect newborn but not adult mice). It may be that the special susceptibility of newborn humans to some viruses, such as Coxsackie B or herpes simplex, is related to the rapid growth rate of the infant’s cells. Decreased interferon production may be observed in young cells in cell cultures and in newborn animals, but the relation of this fact to the increased severity of viral infection in newborn animals is unproved.
- Immature immunologic defenses. IgM antibodies are not usually synthesized by the fetus unless the fetus is exposed to maternal infection. The newborn infant can synthesize IgM antibodies in response to infection but had no antibodies of the IgM type transmitted through the placenta from the maternal circulation. The importance of serum factors such as IgM in protecting the newborn from infection is not clearly established, but such factors probably would be helpful in providing opsonins to aid phagocytosis and other antibodies active against enteric bacteria such as Escherichia coli. Other immature immunologic functions in the newborn period include decreased complement activity, decreased neutrophil chemotaxis, and less effective cell-mediated immunity.
Clinical Approach Infectious Diseases
The study of infectious diseases is different from the study of microbiology. Microbiology concentrates on the study of microorganisms, whereas the discipline of infectious diseases concerns itself with the study of patients. In clinical medicine, the knowledge about microorganism is only part of what is necessary to analyze the observations made of a patient with an infection.
There are two approaches to infectious diseases: the etiologic agent approach and the anatomic syndrome approach.Traditionally, the student’s introduction to infectious diseases is in terms of the particular agent involved. The student learns to identify the characteristics of the infecting organism and the diseases it may cause. However, patients cannot be easily categorized on the basis of etiologic agents, so that when clinical experience begins, the student must shift viewpoints to that of the clinician and think in terms of anatomic syndromes.
These two approaches to infectious diseases can be illustrated by the example of group A streptococci and exudative pharyngitis. In the etiologic agent approach, the various kinds of illnesses that can be caused by group A streptococci are considered. In the anatomic syndrome approach, a broad, general clinical pattern of the syndrome is defined (e.g., febrile exudative pharyngitis). After that, the various microorganisms that may cause this syndrome are evaluated.
Dr. Afsaneh Jeddi