11.1.1—Phagocyte action

Phagocytes are white blood cells that provide an early, non-specific immune response by carrying out phagocytosis: they attach to a pathogen, engulf it and digest it.

1. Chemicals from pathogens and attacked body cells attract a phagocyte; movement towards the stimulus is chemotaxis.
2. Receptor proteins on the phagocyte attach to antigens on the pathogen.
3. The cell-surface membrane extends around the pathogen and encloses it in a phagocytic vacuole, or phagosome; this is endocytosis.
4. The phagosome fuses with a lysosome to form a phagolysosome. Lysosomal digestive enzymes enter the vacuole and destroy the pathogen.

Neutrophils are short-lived phagocytes that patrol tissues and are released in large numbers during infection. Macrophages are longer-lived, settle in organs and carry out phagocytosis without completely destroying the pathogen: they display pathogen antigens on their surface as antigen-presenting cells, allowing lymphocytes to recognise them.

Phagocytosis is a non-specific early defence, not antibody production. Neutrophils digest pathogens completely and then die; macrophages have the additional antigen-presentation role that links phagocytosis to later specific immunity. The detailed primary response and memory-cell sequence belong to later cards.