A host cell is a cell in a human or animal’s body that carries another living organism inside of it or provides the necessities required for the organism to stay alive. This is a very common situation in nature. For example, in humans, there are actually more bacterial cells present inside the body than the number of actual human cells which comprises a person. In many cases, the relationship between the host and the foreign organism is symbiotic, meaning that both entities live together. There are three types of symbiotic host interactions: mutualism, commensalism, and parasitism.
The interaction between a host and a parasite is a relationship that is typically characterized by a power imbalance, with the host providing a source of nourishment and resources for the parasite. The parasite takes advantage of the host’s resources and often causes harm to the host in the process. Parasites may feed on the host’s tissue, interfere with the host’s metabolism, or even cause disease. In some cases, the host may be able to resist the parasite’s effects, while in other cases the host may succumb to the parasite’s activities. In either case, the interaction between a host and a parasite is a complex relationship that can have both positive and negative impacts on the health and well-being of the host.
Mutualism is when both entities benefit from the relationship. One example of this type of interaction is the lactobacilli bacteria which lives in the genitalia of women. These bacteria produce lactic acid that prevents yeast infections and receive glycogen as a form of nourishment. Commensalism is when two organisms are able to live together, but neither provides any discernible benefit to the other. Many relationships are misclassified under commensalism simply because it is not understood exactly how one of the entities is being helped. The last group of interactions is the most dangerous one.
Parasitism, or host parasite interaction, occurs when the foreign organism is nourished from the host’s cells and also causes damage to the host. In humans, the present bacteria can become pathogenic, causing diseases and other complications. These pathogens can be broken down into three groups. The potential, or opportunistic pathogens are those that are of the parasitic kind but are not causing the disease. However, if the host develops some compromising problem or weakness, the bacteria jumps into action causing disease. One such opportunistic pathogen is the bacteria normally found in the mouth that can cause periodontal disease. Other pathogens only exist to cause disease in a host’s system. These are called obligate pathogens. An example of this type is Streptococcus pneumoniae, which is the cause of strep throat.
Once a host parasite interaction has occurred, the body has many ways of reacting, known as the host defense system. In many cases the body’s reactions can neutralize the infection before any noticeable symptoms arise. There are two groups of host defense mechanisms: innate defenses and inducible defenses. The first group are common to all normally healthy patients and exists to protect the body from invasion or colonization by potentially harmful organisms. They are present in many forms and include structural and anatomical barriers, phagocytosis, and inflammation.
In addition, some bacteria that live within the human body help to prevent attacks. Since they are present in the body prior to infection, these reactions are called natural or constitutive resistance. The other type of mechanisms, inducible defenses, occur as a reaction and must be induced. Also known as acquired or adaptive immunity, these occur after exposure to a parasite or pathogen. The body’s immune system is the main constituent of inducible defenses. Other inducible mechanisms include bacterial antigens, antimicrobial agents, and natural antibodies. The host cell interaction can be very complicated, but thorough study of the types of relationships and defenses can be very interesting as well.