Phagocytes and antigens
The first line of defence
- Your immune system is the set of cells that defends against pathogens.
- The first to act are phagocytes — white blood cells like macrophages and neutrophils.
- They recognise what is foreign by its antigens.
Phagocytosis
- A phagocyte destroys a pathogen by phagocytosis:
- it surrounds the pathogen and takes it inside in a vesicle.
- it digests the pathogen with enzymes.
- a macrophage then displays the pathogen's antigen on its surface, to alert other immune cells.
Practice
In phagocytosis, a phagocyte:
A phagocyte engulfs the pathogen into a vesicle and digests it with enzymes.
Practice
After digesting a pathogen, a macrophage:
The macrophage presents the antigen, which activates the next stage of the immune response.
Practice
Phagocytes such as macrophages and neutrophils are types of white blood cell.
Yes — phagocytes are white blood cells; they are the first cells to respond to a pathogen.
Antigens: self and non-self
- An antigen is a molecule (usually a protein) on a cell surface that the immune system can recognise.
- Self antigens are the body's own markers — the immune system ignores them.
- Non-self antigens are foreign (e.g. on a pathogen) — these trigger an immune response.
Practice
Non-self antigens trigger an immune response because they:
Self antigens are ignored; non-self (foreign) antigens are recognised and trigger a response.
You've got it
Key idea
- phagocytes (macrophages, neutrophils) are the first defence
- phagocytosis: engulf → digest with enzymes → display the antigen
- an antigen is a surface molecule the immune system recognises
- self antigens are ignored; non-self (foreign) antigens trigger a response