Immune response

When a pathogen invades the body, the antigens on its cell surface are identified as foreign, which activates cells in the immune system. The antigens are usually proteins or polysaccharides.

The four main stages of the immune response:
1 - Phagocytosis
A phagocyte, eg a macrophage, is a type of white blood cell that carries out phagocytosis (engulfment of pathogens).
They're found in the blood and are the first cells to respond to a pathogen inside the body.
A phagocyte recognises the antigens on a pathogen.
The cytoplasm of the phagocyte moves round the pathogen, engulfing it.
The pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte.
A lysosome (an organelle that contains lysosomal enzymes) fuses with the phagocytic vacuole.
The lysosomal enzymes break down the pathogen.
The phagocyte presents the pathogen's antigens - sticking them to its surface to activate other immune system cells.

2 - T-cell activation
A T-cell is another type of white blood cell.
It has proteins on its surface that bind to the antigens presented to it by the phagocytes.
This activates the T-cell.
Different types of T-cell respond in different ways. Some release substances to activate B-cells. Some attach to antigens on a pathogen and kill the cell.

3 - B-cell activation
B-cells are also a type of white blood cell.
They're covered with antibodies, proteins that bind with antigens to form an antigen-antibody complex.
Each B-cell has a different shaped antibody on its membrane, so different ones bind to different shaped antigens.
When the antibody on the surface of…