Extracellular pathogens spread by direct extension of the focus of infection through the lymphatics or the bloodstream. Usually, spread by the bloodstream occurs only after the lymphatic system has been overwhelmed by the burden of infectious agent.
Obligate intracellular pathogens must spread from cell to cell; they do so either by direct transmission from one cell to the next or by release into the extracellular fluid and reinfection of both adjacent and distant cells. Many common food poisoning organisms cause pathology without spreading into the tissues. They establish a site of infection on the epithelial surface in the lumen of the gut and cause no direct pathology themselves, but they secrete toxins that cause damage either in situ or after crossing the epithelial barrier and entering the circulation.
Most infectious agents show a significant degree of host specificity , causing disease only in one or a few related species. What determines host specificity for every agent is not known, but the requirement for attachment to a particular cell-surface molecule is one critical factor. As other interactions with host cells are also commonly needed to support replication, most pathogens have a limited host range.
The molecular mechanisms of host specificity comprise an area of research known as molecular pathogenesis, which falls outside the scope of this book.
While most microorganisms are repelled by innate host defenses, an initial infection, once established, generally leads to perceptible disease followed by an effective host adaptive immune response. This is initiated in the local lymphoid tissue, in response to antigens presented by dendritic cells activated during the course of the innate immune response Fig. Antigen-specific effector T cells and antibody -secreting B cells are generated by clonal expansion and differentiation over the course of several days, during which time the induced responses of innate immunity continue to function.
Eventually, antigen -specific T cells and then antibodies are released into the blood and recruited to the site of infection Fig. A cure involves the clearance of extracellular infectious particles by antibodies and the clearance of intracellular residues of infection through the actions of effector T cells. After many types of infection there is little or no residual pathology following an effective primary response.
In some cases, however, the infection or the response to it causes significant tissue damage. In other cases, such as infection with cytomegalovirus or Mycobacterium tuberculosis , the infection is contained but not eliminated and can persist in a latent form.
If the adaptive immune response is later weakened, as it is in acquired immune deficiency syndrome AIDS , these diseases reappear as virulent systemic infections.
We will focus on the strategies used by certain pathogens to evade or subvert adaptive immunity and thereby establish a persistent infection in the first part of Chapter In addition to clearing the infectious agent, an effective adaptive immune response prevents reinfection.
For some infectious agents, this protection is essentially absolute, while for others infection is reduced or attenuated upon reexposure. The agents that cause disease fall into five groups: viruses, bacteria , fungi, protozoa, and helminths worms. Protozoa and worms are usually grouped together as parasites, and are the subject of the discipline of parasitology, whereas viruses, bacteria, and fungi are the subject of microbiology.
In Fig. The remarkable variety of these pathogens has caused the natural selection of two crucial features of adaptive immunity. First, the advantage of being able to recognize a wide range of different pathogens has driven the development of receptors on B and T cells of equal or greater diversity. Second, the distinct habitats and life cycles of pathogens have to be countered by a range of distinct effector mechanisms.
These agents act on this penetration and uncoating process. Pleconaril works against rhinoviruses, which cause the common cold, by blocking a pocket on the surface of the virus that controls the uncoating process.
This pocket is similar in most strains of rhinoviruses and enteroviruses, which can cause diarrhea, meningitis, conjunctivitis, and encephalitis.
The problem today with antibiotics or antibacterials, antifungicides, and antiviruses is resistance. Antimicrobial resistance is resistance of a microorganism to an antimicrobial drug that was originally effective for treatment of infections caused by it.
Resistant microorganisms bacteria, fungi, viruses, and parasites are able to withstand attack by antimicrobial drugs so that standard treatments become ineffective and infections persist, increasing the risk of spread to others. The evolution of resistant strains is a natural phenomenon that occurs when microorganisms replicate themselves erroneously or when resistant traits are exchanged between them.
When resistance occurs it is necessary to form modified drugs to avoid resistance. For instance, ampicillin and amoxicillin are variants of penicillin to work around resistance of common infections. Know these keywords and what each word refers to as well as its function.
Read over the list. Are all these keywords familiar? Answer these questions. Do you know the answers, holy cow! Check yourself out in this quiz, and prove that you are a humanology wiz. National Center for Biotechnology Information , U. Human Physiology, Biochemistry and Basic Medicine. Published online Oct Laurence Cole and Peter R. Many viruses, when released from infected cells, will be effectively knocked out by antibodies that have been produced in response to infection or previous immunisation.
Antibiotics are useless against viral infections. This is because viruses are so simple that they use their host cells to perform their activities for them. So antiviral drugs work differently to antibiotics, by interfering with the viral enzymes instead. Antiviral drugs are currently only effective against a few viral diseases, such as influenza, herpes, hepatitis B and C and HIV — but research is ongoing.
A naturally occurring protein, called interferon which the body produces to help fight viral infections , can now be produced in the laboratory and is used to treat hepatitis C infections. It is possible to vaccinate against many serious viral infections such as measles, mumps, hepatitis A and hepatitis B. An aggressive worldwide vaccination campaign, headed by the World Health Organization WHO , managed to wipe out smallpox.
However, some viruses — such as those that cause the common cold — are capable of mutating from one person to the next. This is how an infection with essentially the same virus can keep dodging the immune system. Vaccination for these kinds of viruses is difficult, because the viruses have already changed their format by the time vaccines are developed. This page has been produced in consultation with and approved by:. Anthrax is a rare but potentially fatal bacterial disease that occasionally infects humans.
The Western obsession with cleanliness may be partly responsible for the increase in allergic asthma and conditions such as rhinitis. Careful prescribing of antibiotics will minimise the emergence of antibiotic resistant strains of bacteria. Aspergillus is a fungus that commonly grows on rotting vegetation. It can cause asthma symptoms. The simplest form of prevention for lyssavirus is to avoid close contact with bats. Content on this website is provided for information purposes only.
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