What are antigens?

Antibody-mediated immunity and cell-mediated immunity are both triggered by antigens, which are any substance capable of causing a specific immune response. They are large molecules such as proteins, carbohydrates, lipids, or nucleic acids and may include things such as virus particles, whole micro-organisms (such as a bacterial cell), part of a bacterium (such as the flagella, cell wall or capsule), toxins, molecules on cells such as blood cells, pollen grains and egg whites.

Large molecules produced in a person’s own body, called self-antigens, do not cause an immune response. Foreign compounds that do trigger an immune response are non-self antigens

The immune system becomes programmed prior to birth to distinguish between self-antigens and non-self antigens, and from then on, it only attacks non-self antigens.

What are antibodies?

An antibody is a Y-shaped specialised protein that is produced by plasma cells in response to a non-self antigen. Antibodies belong to a group of proteins known as immunoglobulins, often represented as ‘Ig’.

There are five classes of antibodies, which vary in their structure and are designated;

• IgA

• IgD

• IgE

• IgG

• IgM

The antibody produced in response to an antigen can combine with that antigen to form an antigen-antibody complex. Each antibody can combine with only one particular antigen.

What are antigen-presenting cells?

When a non-self antigen enters a body, specific cells recognise this and respond appropriately. These cells are called antigen-presenting cells, and include dendritic cells, macrophages and undifferentiated B-cells – these cells:

• Detect the presence of a non-self-antigen

• Engulf the pathogen

• Digest the pathogen, producing small fragments that move to the surface of the cell

• Present the antigen to lymphocytes