Immunologic variants in children
The human defense system is built to allow human life to have a successful healthy progression from the mother’s womb all the way up to adulthood. This process has defined immunological steps. The many differences between an adult and a neonate will be discussed throughout this work.
First, let’s begin with the development of the cells involved in defense. At 7 weeks of gestation most of the cells in the liver are hematopoietic (60 percent). In the liver these may differentiate into neutrophills, hepatic white blood cells among other (figure 1). However, after the 24th of gestation the bone marrow becomes (and remains in adults) the major site of hematopoiesis. At the beginning, the fetal bone marrow has equal number of myeloid and erythroid cells. “However, myeloid cells predominate by 12 weeks' gestation and the myeloid-to-erythroid ratio approaches the adult level of 3:1 by 21 weeks' gestation”(1). Lymphopoesis occurs in the lymph plexuses and the thymus after 9 weeks’ gestation. “B cells with surface IgM are present in the liver, and circulating lymphocytes also are seen at 9 weeks' gestation. T lymphocytes are found only rarely before 12 weeks' gestation and 31 Lymphocyte subpopulations are detected by 13 weeks'
gestation in fetal liver.
The human defense system is built to allow human life to have a successful healthy progression from the mother’s womb all the way up to adulthood. This process has defined immunological steps. The many differences between an adult and a neonate will be discussed throughout this work.
First, let’s begin with the development of the cells involved in defense. At 7 weeks of gestation most of the cells in the liver are hematopoietic (60 percent). In the liver these may differentiate into neutrophills, hepatic white blood cells among other (figure 1). However, after the 24th of gestation the bone marrow becomes (and remains in adults) the major site of hematopoiesis. At the beginning, the fetal bone marrow has equal number of myeloid and erythroid cells. “However, myeloid cells predominate by 12 weeks' gestation and the myeloid-to-erythroid ratio approaches the adult level of 3:1 by 21 weeks' gestation”(1). Lymphopoesis occurs in the lymph plexuses and the thymus after 9 weeks’ gestation. “B cells with surface IgM are present in the liver, and circulating lymphocytes also are seen at 9 weeks' gestation. T lymphocytes are found only rarely before 12 weeks' gestation and 31 Lymphocyte subpopulations are detected by 13 weeks'
gestation in fetal liver. Figure 1immune system development (differentiation)
Second, the function and mechanism of the defense cells also varies as the fetus transforms to a child and then an adult. “Cellular defense mechanisms and humoral immunity of the newborn differ from those found later in life, and these undoubtedly contribute to the unusual susceptibility to infection noted in the neonatal period.” For example, monocytes in newborns are slower than monocytes in adults because they have reduced ATP production and phagocytosis. Because of these functional differences newborns are more susceptible to many kinds of infection agents.
Third, as an infant grows into an older person, some variants are observed in their immune system, specifically the cell count of white blood cells which does not necessarily mean that there is something wrong with the defense system of the developing body. For instance, higher ranges for white blood cell counts are observed among children, newborns and infants compared to adult values (2). The physician must keep in mind that these values are absolutely normal when reading the lab results of an infant patient for that this does not represent an infection (3). We must add that for the first 4-6 months of the newborn’s life, those whom are breast feed receive immunity passively conveyed by the mother that contains the immunoglobulins (Ig G and Ig A particularly) that protects to the infants against invaders during first year of life.
As we progress in age and become elderly, there is a deterioration of the immune system, including the atrophy of the thymus that compromises the defense system of the human body and causes significant variations between the immune system of children and the elderly1. Genetic mutations throughout the course of human development also cause variations at the molecular level between children and older patients that reflect on differences of the immune system among different age groups that trigger immunodeficiency or autoimmune disorders. All these differences allow us to marvel at the complexity of our immune system.
Multiple Choice Questions
1. Where is hematopoeisis taking place at week 7 of gestation?
a. Bone marrow
b. Thymus
c. Liver
d. Spleen
2. Why are monocytes in adults faster than in newborns?
a. amount of mitochondria
b. increased ATP production
c. amount of bone marrow
3. Which type of defense do children have during first year of life?
a. innate immunity and maternal immunoglobulins
b. only innate immunity
c. plasma cells
a. innate immunity and maternal immunoglobulins
b. only innate immunity
c. plasma cells
Discussion Questions
4. Discuss the consequences of an adult having monocytes like a newborn.
4. Discuss the consequences of an adult having monocytes like a newborn.
5. Do the differences between children and adult ranges in WBC count point to a microbial insult or not? Explain.
6. What happen if someone does not have passive immunity? Hypothetically, what is treatment you chose?
6. What happen if someone does not have passive immunity? Hypothetically, what is treatment you chose?
References
1. Segel GB., Palis J. Chapter 6. Hematology of the Newborn. In: Lichtman MA, Beutler E, Kipps T, eds. Williams Hematology. USA: The McGraw-Hill Companies: 2006
2. Dales Nursing Place: Common Lab Values. Available at: http://www.dalesplace.net/lab_values.php#Hematology%20Values. Accessed May 21, 2008
3. Siparsky G, Accurso FJ, Chapter 43. Chemistry & Hematology Reference Intervals. In: Hay W, Levin M, Sondheimer J, eds. Current Pediatric Diagnosis and Treatment. USA: The McGraw-Hill Companies: 2006
Figure 1 obtained at http://espanol.images.search.yahoo.com/images/view?back=http%3A%2F%2Fespanol.images.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dimmune%2Bsystem%26fr%3Dyfp-t-340%26ei%3Dutf-8%26js%3D1%26x%3Dwrt&w=400&h=400&imgurl=www.drstandley.com%2Fimages%2Fimmune.bmp&rurl=http%3A%2F%2Fwww.drstandley.com%2Fbodysystems_immune.shtml&size=157.3kB&name=immune.bmp&p=immune%20system&type=bmp&oid=3ced6bf09a97e724&no=1&tt=67.294
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