Non-specific Animal Defences: Phagocytosis Revision notes | A-Level Biology OCR

Non-specific Animal Defences: Phagocytosis

This lesson covers:

The role of phagocytes in the immune response
The process of phagocytosis

Phagocytes

Phagocytosis is a type of non-specific defence involving a type of cell known as a phagocyte.

Flowchart showing non-specific and specific defence mechanisms including phagocytosis, cellular response, and humoral response.

Phagocytes are a type of white blood cell that engulf and destroy pathogens (phagocytosis). They are found in the blood and body tissues of many organisms.

There are two main types of phagocyte:

Neutrophils - These rapidly engulf and destroy pathogens at the site of infection.
Macrophages - These engulf and digest pathogens but also present the pathogen's antigens on its cell surface to activate other cells in the immune system.

Phagocytosis

The process of phagocytosis is carried out as follows:

The pathogen releases chemicals that attract a phagocyte.

Diagram illustrating the process of phagocytosis showing a phagocyte and a pathogen.

The phagocyte recognises the pathogen's antigens as non-self. This causes the phagocyte to bind to the pathogen.

Diagram showing the process of phagocytosis involving a phagocyte and a pathogen.

The phagocyte engulfs the pathogen.

Illustration showing a phagosome containing a pathogen inside a phagocyte.

The pathogen is now contained within a vacuole known as a 'phagosome'.

Diagram showing the process of phagocytosis with a phagolysosome and lysosome.

The lysosome, containing lysozymes, fuses with the phagosome to form a phagolysosome.

Diagram showing a phagocyte with a pathogen inside.

Lysozymes digest and destroy the pathogen.

The phagocyte presents the pathogen's antigens on its surface to activate other cells in the immune system. The phagocyte is then referred to as an antigen-presenting cell (APC).

All stages of phagocytosis.

The diagrams above only show one phagocyte and one pathogen, but in reality many phagocytes and many pathogens interact at the same time.

Cytokines and opsonins

Cytokines and opsonins are molecules that help with the process of phagocytosis.

Cytokines are chemicals released by phagocytes that have engulfed a pathogen. They act as cell-signalling molecules to trigger the movement of other phagocytes to the site of infection. Cytokines also trigger an increase in body temperature which inhibits the reproduction of pathogens and allows the specific immune system to work faster.

Opsonins are chemicals (e.g. antibodies) that bind to pathogens to make them easily recognisable by phagocytes. Phagocytes contain receptors on their cell-surface which bind to common opsonins, making it easier for the phagocyte to bind to the pathogen and destroy it.