Phagocytes and Blood Smears 🩸🔬

What are Neutrophils and what is their function as well as their structure?

• Appearance: They have a multi-lobed nucleus (often 3–5 lobes).

• Role: They are the "first responders" to infection. They engulf and destroy pathogens rapidly but are short-lived.

• Action: They ingest pathogens into a phagosome, fuse it with a lysosome, and digest it.

What are Macrophages and what is their function as well as their structure?

• Appearance (Monocytes): Much larger cells with a kidney-bean shaped nucleus.

• Role: They arrive later but last longer. They act as Antigen Presenting Cells (APCs).

• Action: After digesting the pathogen, they save the antigen and display it on their surface to activate T-cells.

Describe the appearance of a Neutrophil in a blood smear.

• It has a multi-lobed nucleus (segmented) and granular cytoplasm.

Describe the appearance of a Monocyte/Macrophage in a blood smear.

• It is a very large cell with a huge, kidney-bean shaped (non-lobed) nucleus.

What is a Phagosome?

• A vesicle formed inside a phagocyte when it engulfs a pathogen.

What is the role of Lysosomes in phagocytosis?

• They fuse with the phagosome and release hydrolytic enzymes (lysozymes) to digest/destroy the pathogen.

What is an Opsonin?

• A molecule (such as an antibody) that binds to a pathogen and facilitates the binding of the phagocyte to the pathogen.

What is an Antigen Presenting Cell (APC)?

• A phagocyte (usually a macrophage) that, after destroying a pathogen, displays the pathogen's antigens on its cell surface to activate other immune cells.

What is the role of Cytokines?

• They are cell-signalling molecules (secreted by T-cells) that help coordinate the immune response, for example by stimulating B-cells to mature and multiply.

Which type of phagocyte is usually the first to arrive at a site of infection?

• Neutrophils.

Describe the sequence of events that occurs after a pathogen is engulfed by a phagocyte. (4 marks)

• The pathogen is enclosed in a vesicle called a phagosome.

• A lysosome fuses with the phagosome.

• Hydrolytic enzymes / lysozymes are released into the phagosome, creating a phagolysosome.

• The pathogen is digested / hydrolysed into small molecules.

Explain the role of an Antigen Presenting Cell (APC) in the specific immune response. (3 marks)

• After engulfing a pathogen, the APC displays the antigen on its cell surface.

• This allows T-cells (T-helper cells) to recognise the antigen as foreign.

• This recognition activates the T-cells to stimulate the specific immune response (e.g., clonal expansion).

Explain how opsonins increase the efficiency of phagocytosis. (2 marks)

• Opsonins (such as antibodies) bind to the antigens on the surface of the pathogen.

• They provide binding sites for the phagocyte / facilitate the binding of the phagocyte to the pathogen, making engulfment easier.