Disease production by pathogens

The first stage of disease production by pathogens is infection, or entry of the pathogen into the body. Nearly all pathogens must gain entry into the body before they can begin to replicate or spread. A few pathogens can exist on the skin (e.g. viruses causing warts) or in the gut (e.g. bacteria causing cholera) without technically entering the body. However, infection is not made easy for pathogens because the body has many physical and chemical barriers to try to prevent pathogens entering the body.

Stages of disease production by pathogens

Size is not the only way in which infectious organisms vary. They also vary enormously with respect to how they enter and live within the body and actually cause disease. Infection and disease production by pathogenic organisms can be divided into four stages:
1. Invasion.
2. Multiplication.
3. Spread.
4. Production of disease (pathogenesis).

Although infection usually involves all of these steps, there are many exceptions in terms of both the steps involved and their order. Some pathogens do not spread significantly or even technically gain entry to the body. Organisms may replicate locally before spreading or may spread through the body before beginning significant replication. Pathogens show considerable variation at each of these stages of infection, as will be described below.

Size of pathogens

One feature of the range of pathogenic organisms listed in table is the enormous variation in size. Viruses are the smallest infectious organisms, being 20–400 nm in size. At the other end of the scale some parasitic worms, such as the tapeworm, can be up to 7 m (20 ft) in length. This represents a difference in scale of a factor of 109. To put that into some sort of perspective, if a virus were the size of a tennis ball, a fully developed tapeworm would reach from London to Los Angeles. It does not stretch the imagination too far to appreciate that the problems posed to the immune system by these two organisms would require very different solutions.

Types of pathogen and how they differ

The types of pathogen that can cause disease include many groups of single-celled microorganisms and larger multicellular parasites. Viruses, bacteria, some yeasts, and protozoan parasites are examples of singlecelled pathogens. Fungi and helminths (parasitic worms) are the major multi-cellular pathogens. These pathogens come from very different parts of the biological kingdom and vary considerably in many aspects. Pathogens differ enormously in their size. They also have very different lifestyles and cause disease in a variety of ways.

Size and lifestyle of pathogens

Why is immunology so complicated?

There are thousands of components to the immune system, and during the course of learning about some of these it can appear that the immune system is far more complicated than necessary for achieving what is, on the surface, the simple task of eliminating an infectious organism. There are a number of reasons why the immune system is complex. The first of these is the desirability of eliminating pathogens without causing damage to the host. Getting rid of a pathogen is theoretically easy. If you had an infection in your liver you could produce a nasty toxin that would kill the pathogen; unfortunately it would also destroy your liver. Killing pathogens is not difficult, but getting rid of pathogens without damaging the host is much more complicated. Imagine if a city in your country was infiltrated by soldiers from another country; you could get rid of the foreign invaders by dropping a nuclear bomb on the city but this would also kill a lot of your compatriots. To go into the city to eliminate or capture the foreign soldiers without causing harm to your compatriots is much more difficult. This analogy raises a major issue concerning the immune system – that of recognition. To eliminate foreign soldiers without killing your own requires that you can tell the two apart. In the same way the immune system must be able to distinguish between pathogens and host cells so that it can direct its destructive powers towards the pathogens. Many of the specialised features of the immune system are involved in recognition of foreign pathogens.
An additional problem facing the immune system is that pathogens come in all shapes and sizes, with different lifestyles and different ways of causing disease. To understand fully the complexity that the immune system must deal with, it is necessary to have some understanding of infectious organisms and the ways in which they cause disease. The rest of this chapter describes how pathogens differ, so that it is possible to get an appreciation of the problems faced by the immune system.

The role and complexity of the immune system

The immune system consists of proteins, cells and organs that are concerned with defence of the individual, primarily against the threat of disease caused by infectious organisms. An infectious organism that causes disease is called a pathogen and the individual (person or animal) that is infected by a pathogen is called the host. Not all infectious organisms cause disease and some are actually beneficial, for example bacteria living in the gut help to digest certain foods. Infectious organisms that help the host are called commensal organisms. However, many viruses, bacteria, fungi, yeasts and parasites are pathogenic and we are constantly in danger of infection and disease caused by them.

 

Some components of the immune system. The bone marrow and thymus are
sites of production of lymphocytes, white blood cells involved in immune responses.
Adenoids, tonsils, lymph nodes, spleen and lymphoid nodules are sites where immune
responses are generated. Lymphatic vessels are similar to blood vessels and allow cells of the
immune system to travel throughout the body.

The importance of the immune system in resistance to infection is demonstrated by the fact that, without treatment, babies born with severe defects of certain components of the immune system die in infancy of overwhelming infection, usually with organisms (e.g. Haemophilus influenzae, Candida albicans or cytomegalovirus) that cause little or no disease in individuals with a normal immune system. More recently, the AIDS (acquired immunodeficiency syndrome) epidemic has brought the consequences of having a poorly functioning immune system to the attention of the public at large. AIDS patients usually die from infection, such as with the yeasts Pneumocystis carinii, which causes pneumonia, or Cryptococcus neo-formans, a cause of meningitis. The immune system normally controls these infections with little or no damage to the host.
The immune system is primarily involved in defence against infectious organisms, but it also plays a role in various other diseases. Evidence is accumulating that the immune system can provide protection against some tumours. Exciting new developments also suggest that immune responses can be induced against tumours that normally do not provoke an immune response. These so-called tumour vaccines offer hope of additional weapons in the armoury against cancer. The immune system is also responsible for the rejection of transplants. In this instance the immune system is acting normally in trying to defend the body against a foreign invader, even though the ‘invader’ is beneficial. Rejection of transplants is a major cause of graft loss and much effort is being devoted to try to prevent transplant rejection.
Although a properly functioning immune system is essential for good health, like any other physiological system the immune system can malfunction and actually cause diseases. The most common form of disease caused by the immune system is allergy, for example hayfever and food allergy, which is caused by an inappropriate immune response against generally harmless material such as pollen or food. Another category of diseases caused by malfunction of the immune system are the autoimmune diseases, including rheumatoid arthritis and some types of diabetes. Autoimmune diseases occur when the immune system attacks the body’s own tissue. Although these diseases caused by the immune system can be threatening and sometimes fatal, on balance a properly functioning immune system is essential for life in the world we inhabit, which is full of disease-causing organisms, some of which, as mentioned above, are invariably fatal if the system does not operate properly.