1.1  Immunity and the immune system

Immunity is a state in which the body has sufficient defences to be able to resist the development of communicable diseases caused by infectious agents. The main types of infectious agents are bacteria, viruses, fungi, protozoa and parasites. They are also often referred to as pathogens, which means ‘disease-causing organisms’. We will use both terms in this Module.

  • Which of the following are infectious agents: the hepatitis B virus, polio virus, Mycobacterium tuberculosis, Neisseria gonorrhoeae, Candida albicans, Giardia intestinalis, and Plasmodium falciparum?

  • All of these are infectious agents. The list includes two viruses (causing hepatitis and polio), two bacteria (causing tuberculosis and gonorrhoea), a fungus (Candida causes oral and genital thrush), one protozoan (Giardia causes diarrhoea), and one parasite (Plasmodium causes malaria).

The immune system is the name given to the network of cells, proteins, tissues and organs within the body (Figure 1.1), which act together to protect us against infectious agents. In addition to the structures shown in Figure 1.1, the cells of the immune system also circulate in the blood and some of them migrate through the tissues. These cells are usually known as white blood cells, which is a confusing name because they are found throughout the body – not just in the blood. Wherever an infectious agent gets into the body, it will soon be detected and attacked by the immune system.

Figure 1.1  The sites in the body (in addition to the blood) where the cells and molecules of the human immune system are concentrated. (Source: The Open University, SXR376 Preparatory Reading, Figure 1.2)

The immune system of a healthy and well-nourished adult may be able to fight an infection and stop the disease from developing, or reduce it to mild symptoms. But in very young or elderly persons, or people who are malnourished or in bad health – particularly if they already have HIV, TB or malaria – the immune system is not strong enough to protect them from a new infection. They can become very ill and even die without medical treatment.

  • In addition to the immune system, can you think of other ways in which the human body protects itself from infectious agents?

  • Intact skin covering our bodies acts as a barrier preventing entry of infectious agents. You may have also thought of the hairs and mucus inside the nose, which trap bacteria from the air. Coughing and sneezing, vomiting and diarrhoea rids the body of large numbers of infectious agents, but also spreads them to other people.

Learning Outcomes for Study Session 1

1.1.1  How does the immune system protect us from infection?