What is the primary difference between a phagosome and a phagolysosome?

A phagosome is a vesicle containing only the engulfed pathogen, while a phagolysosome is formed when a phagosome fuses with a lysosome, allowing digestive enzymes to enter and destroy the pathogen.

How do the lifespans and fates of neutrophils and macrophages differ after phagocytosis?

Neutrophils are short-lived and typically die after engulfing a few pathogens (forming pus), whereas macrophages are long-lived and can continue to function and present antigens to other immune cells.

What is the functional difference between a monocyte and a macrophage?

Monocytes are the precursor cells found circulating in the blood; once they migrate into tissues and organs, they differentiate into larger, more active macrophages.

What is a common misconception regarding the specificity of phagocytes?

A common error is thinking phagocytes target specific diseases like a vaccine does; in reality, they are non-specific and will attempt to engulf any foreign particle or pathogen they recognize as 'non-self'.

Why is it incorrect to say that neutrophils initiate the specific immune response?

Neutrophils focus entirely on destroying pathogens and then dying; they do not act as Antigen Presenting Cells (APCs). Only macrophages (and dendritic cells) present antigens to lymphocytes to start the specific response.

What mistake is often made when describing the role of the nucleus in phagocytosis?

Students often think the nucleus digests the pathogen. In fact, the nucleus only contains genetic information; the digestion is carried out by enzymes stored in the lysosomes.

Define 'Chemotaxis' in the context of the immune system.

Chemotaxis is the movement of a cell (like a neutrophil) toward a higher concentration of a chemical stimulus, such as cytokines or bacterial waste products, to locate an infection.

What are 'Pseudopodia'?

Pseudopodia are temporary, arm-like extensions of the phagocyte's cell membrane used to surround and 'grab' a pathogen during the ingestion phase of phagocytosis.

What does the term 'Antigen Presenting Cell' (APC) mean?

An APC is a cell, like a macrophage, that digests a pathogen and then displays fragments of the pathogen's antigens on its own surface to be recognized by T-lymphocytes.

Why are lysosomes essential for the function of a phagocyte?

Lysosomes contain hydrolytic enzymes (like proteases) that break down the biological molecules of the pathogen; without them, the phagocyte could engulf but not kill the invader.

Phagocytes | Cambridge International Examinations AS-Level Biology

Revision Notes

AS-Level

Cambridge International Examinations

Biology

11. Immunity

Phagocytes

Summary

Phagocytes are specialized white blood cells (leukocytes) that provide a non-specific, innate immune defense by ingesting and destroying pathogens, dead cells, and foreign debris. They originate in the bone marrow and operate through a multi-step process called phagocytosis, with different types like neutrophils and macrophages serving distinct roles in immediate response and long-term immune signaling.

1. Definition & Core Concepts

Phagocytes are a group of white blood cells produced continuously in the bone marrow and distributed via the bloodstream to tissues throughout the body.

They are the primary effectors of the innate immune response, meaning they act non-specifically against any foreign material rather than targeting specific strains of pathogens.

Their primary function is phagocytosis, a form of endocytosis where the cell engulfs large particles to neutralize potential threats and clear cellular waste.

The two most significant types of phagocytes in the human immune system are neutrophils and macrophages, each possessing unique lifespans and physiological roles.

Diagram of a phagocyte extending pseudopodia to engulf a pathogen, showing internal structures like the nucleus and lysosomes.

2. The Phagocytosis Process

1. Chemotaxis and Adherence

Phagocytes move toward the site of infection by following a chemical gradient (cytokines or bacterial toxins), a movement known as chemotaxis.
Once at the site, receptor proteins on the phagocyte's membrane bind to specific markers (antigens) on the pathogen's surface.

2. Ingestion (Endocytosis)

The phagocyte extends its membrane in projections called pseudopodia to surround the pathogen.
The pathogen is eventually enclosed within a specialized internal vesicle known as a phagosome (or phagocytic vacuole).

3. Digestion and Killing

Lysosomes within the phagocyte fuse with the phagosome to create a phagolysosome.
Digestive enzymes (such as proteases) and reactive oxygen species are released into the vacuole to break down and destroy the pathogen.

3. Types of Phagocytes

Neutrophils: These are the most abundant white blood cells and act as 'first responders.' They are short-lived, travel rapidly through the blood, and can squeeze through capillary walls to reach infected tissues. After ingesting a few pathogens, they typically die, forming the primary component of pus.
Macrophages: Larger and longer-lived than neutrophils, these cells develop from monocytes that leave the blood to settle in organs like the lungs, liver, and spleen. They do not die immediately after phagocytosis and play a critical role in the specific immune response.

4. Key Distinctions

5. Exam Strategy & Tips