Granulocytes: Defenders of the Body
Granulocytes are a category of white blood cells characterised by the presence of granules in their cytoplasm. These granules contain various enzymes and other substances that play a crucial role in the immune response. Granulocytes, along with erythrocytes and platelets, are key components of blood, a specialised connective tissue vital for oxygen transport, nutrient delivery, waste removal, and immune defence. There are three main types of granulocytes: neutrophils, eosinophils, and basophils.
Neutrophils: The First Responders
Neutrophils, also called polymorphonuclear leukocytes (PMNs) or just polymorphs, are the most abundant type of granulocyte, making up 57–67% of circulating leukocytes. They are the first responders to infection, rapidly migrating to sites of inflammation where they actively pursue bacterial cells using chemotaxis.
Distinguishing features of neutrophils:
- Multi-lobed nucleus (polymorphonuclear): Typically, the nucleus has 2–5 lobes (usually 3), connected by thin strands of chromatin. In females, an inactive X chromosome can appear as a drumstick-like appendage on one of the lobes.
- Fine, neutral-staining granules: The cytoplasm contains two main types of granules:
- Azurophilic granules (primary granules): These are lysosomes containing enzymes such as myeloperoxidase and lysosomal hydrolases that are involved in killing and digesting engulfed microorganisms.
- Specific granules (secondary granules): These are smaller and less dense than azurophilic granules and contain various enzymes, including collagenases, that degrade extracellular matrix components. They also deliver additional bactericidal proteins to phagolysosomes and insert new components into the cell membrane.
- Size: Neutrophils are 12–15 μm in diameter in blood smears, about twice the size of erythrocytes.
Functions of neutrophils:
- Phagocytosis: Neutrophils are highly effective at phagocytosis, the process of engulfing and destroying pathogens like bacteria. They use pseudopodia to trap bacteria and internalise them in vacuoles called phagosomes. Specific granules then fuse with phagosomes, releasing their contents, followed by azurophilic granules discharging their enzymes into the acidified vesicle, effectively killing and digesting the microorganism.
- Release of antimicrobial substances: Neutrophils release various antimicrobial substances, including defensins and lysozyme, which contribute to the innate immune response.
- Inflammatory response: Neutrophils play a key role in the inflammatory response, releasing chemokines and cytokines that attract other immune cells to the site of infection.