Title: Immunity and the immune system
Course: Level 3 - Immunization > Section: Immunity, vaccine and expanded program on immunisation

Immunity is a state in which the body has sufficient defence 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 referred to as pathogen, which means "disease causing organisms". We will use both terms in this module

- Question: Which one of the following is an infectious agents?: Poliovirus, Mycobacterium tuberculosis, Plasmodium falciparum, Hepatitis B virus, Neisseria gonorrhoeae, Candida albicans and Giardia intestines?
- Answer: All of them are infectious agents. The list includes two bacteria (causing tuberculosis and gonorrhoea), two viruses (causing Hepatitis and Polio), 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. Organs of the immune system are positioned throughout the body.

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.