The sequence of an immune reaction starts with the recognition of an invader such as bacteria, viruses, or fungi.

These pathogens have specific structures on their surface, called antigens.

Immune system cells like Macrophages or Dendritic Cells are present in the body and recognize these antigens as foreign. They take up the pathogen, break it apart, and present the antigens like a warning sign on their cell surface.

This presentation alerts other immune system cells.

This process ensures that the body can distinguish between own and foreign structures to enable a precise defense.