8 Immunity: defence against disease

Immunity Infection is entry into the body of a micro-organism that may cause disease. Infection does not necessarily lead to disease.

The immune system has two kinds of response to the entry of foreign material. One response involves natural or innate immunity, which is non-specific. The other response involves acquired or adaptive immunity, which is highly specific. Specific immunity involves the production of specialised cells and chemical substances known as antibodies which act against a particular infection.

Non-specific immunity The defence system of the body is called the immune system. The immune system is able to identify ‘non- self’ material from ‘self’. The immune system can produce two kinds of response to invading foreign material, namely non-specific and specific.

Non-specific immunity Micro-organisms are prevented from entering the body by a number of non-specialised features. This is the first line of defence. There is a second line of defence against foreign material that enters the body.

Specific immunity If non-specialised defences fail to prevent infection, specialised responses occur. All cells have protein markers on their surfaces. Non-self markers on cells entering a person are called antigens.

Specific immunity A number of different kinds of cell are involved in specific immunity. The phenotype is the physical, biochemical or physiological expression of the genotype. Some cells produce antibodies that circulate in body fluids and react with specific antigens.

Different kinds of antibodies The immune response produces different kinds of T cells. Different kinds of T cells have different functions. The immune system contains many lymphoid organs and tissues.

Acquiring specific immunity

Specific immunity can be acquired in different ways. In actively acquired immunity, the immune system of a person produces antibodies in response to antigens. In passively acquired immunity, a person receives antibodies from an outside source. Both active and passive immunity can be acquired naturally or artificially.

Adverse events associated with immunity Any part of the immune response can be faulty. Cells of the immune system are involved in allergic reactions. An immune system can lose the ability to distinguish ‘self’ from ‘non-self’.

Adverse events associated with immunity The action of the immune system can be reduced by treatment with certain drugs. Antibodies are sometimes injected to inhibit an immune response. Most plants resist infection by mechanical and chemical means