- a) A parasite is defined as an organism of one species living in or on an organism of another species (a heterospecific relationship) and deriving its nourishment from the host (is metabolically dependent on the host). (See Cheng, T.C., General Parasitology, p. 7, 1973.)
b) A human embryo or fetus is an organism of one species (Homo sapiens) living in the uterine cavity of an organism of the same species (Homo sapiens) and deriving its nourishment from the mother (is metabolically dependent on the mother). This homospecific relationship is an obligatory dependent relationship, but not a parasitic relationship.
- a) A parasite is an invading organism -- coming to parasitize the host from an outside source.
b) A human embryo or fetus is formed from a fertilized egg -- the egg coming from an inside source, being formed in the ovary of the mother from where it moves into the oviduct where it may be fertilized to form the zygote -- the first cell of the new human being.
- a) A parasite is generally harmful to some degree to the host that is harboring the parasite.
b) A human embryo or fetus developing in the uterine cavity does not usually cause harm to the mother, although it may if proper nutrition and care is not maintained by the mother.
- a) A parasite makes direct contact with the host's tissues, often holding on by either mouth parts, hooks or suckers to the tissues involved (intestinal lining, lungs, connective tissue, etc.).
b) A human embryo or fetus makes direct contact with the uterine lining of the mother for only a short period of time. It soon becomes isolated inside its own amniotic sac, and from that point on makes indirect contact with the mother only by way of the umbilical cord and placenta.
- a) When a parasite invades host tissue, the host tissue will sometimes respond by forming a capsule (of connective tissue) to surround the parasite and cut it off from other surrounding tissue (examples would be Paragonimus westermani, lung fluke, or Oncocerca volvulus, a nematode worm causing cutaneous filariasis in the human).
b) When the human embryo or fetus attaches to and invades the lining tissue of the mother's uterus, the lining tissue responds by surrounding the human embryo and does not cut it off from the mother, but rather establishes a means of close contact (the placenta) between the mother and the new human being.
- a) When a parasite invades a host, the host will usually respond by forming antibodies in response to the somatic antigens (molecules comprising the body of the parasite) or metabolic antigens (molecules secreted or excreted by the parasite) of the parasite. Parasitism usually involves an immunological response on the part of the host. (See Cheng, T.C., General Parasitology, p. 8.)
b) New evidence, presented by Beer and Billingham in their article, "The Embryo as a Transplant" (Scientific American, April, 1974), indicates that the mother does react to the presence of the embryo by producing humoral antibodies, but they suggest that the trophoblast -- the jacket of cells surrounding the embryo -- blocks the action of these antibodies and therefore the embryo or fetus is not rejected. This reaction is unique to the embryo-mother relationship.
- a) A parasite is generally detrimental to the reproductive capacity of the invaded host. The host may be weakened, diseased or killed by the parasite, thus reducing or eliminating the host's capacity to reproduce.
b) A human embryo or fetus is absolutely essential to the reproductive capacity of the involved mother (and species). The mother is usually not weakened, diseased or killed by the presence of the embryo or fetus, but rather is fully tolerant of this offspring which must begin his or her life in this intimate and highly specialized relationship with the mother.
- a) A parasite is an organism that, once it invades the definitive host, will usually remain with host for life (as long as it or the host survives).
b) A human embryo or fetus has a temporary association with the mother, remaining only a number of months in the uterus.