6.(b) Describe characteristic features and specific functions of lymphocytes, monocytes and neutrophils.

Lymphocytes, monocytes, and neutrophils are distinct types of white blood cells, each with specialized roles in the immune system. They differ in their morphology, abundance, and primary functions, collectively ensuring a robust defense against pathogens and cellular debris.

1. Lymphocytes

Lymphocytes are a type of agranulocyte and are central to adaptive immunity, providing specific and long-lasting protection. They originate from stem cells in the bone marrow and mature in different lymphoid organs.

Characteristic Features:

• Size: Typically small, ranging from 6-9 µm in diameter, though larger activated lymphocytes can be 10-14 µm.
• Nucleus: Possess a large, spherical nucleus that stains deeply and occupies most of the cell's volume. It often shows a clear perinuclear zone or halo.
• Cytoplasm: Scant cytoplasm, appearing as a thin rim around the nucleus, usually basophilic (pale blue/purple staining) and lacking visible granules.
• Abundance: Constitute about 20-40% of the total white blood cell count in adults.
• Lifespan: Most are short-lived (weeks to months), but some memory cells can live for years.

Specific Functions:

• B Lymphocytes (B cells): Responsible for humoral immunity. Upon activation by an antigen, B cells differentiate into plasma cells, which produce and secrete large quantities of antibodies. These antibodies neutralize pathogens (e.g., viruses, bacteria) and mark them for destruction by other immune cells. Some B cells become memory B cells, providing immunological memory.
• T Lymphocytes (T cells): Involved in cell-mediated immunity. T cells mature in the thymus and recognize specific antigens presented on the surface of infected or abnormal cells.
  • Helper T cells (CD4+ T cells): Coordinate immune responses by activating other immune cells, including B cells and cytotoxic T cells, through the release of cytokines.
  • Cytotoxic T cells (CD8+ T cells): Directly attack and kill virus-infected cells, cancer cells, and foreign cells (e.g., in transplanted organs) by inducing apoptosis.
  • Regulatory T cells: Suppress excessive immune responses to prevent autoimmunity.
• Natural Killer (NK) cells: While technically innate lymphoid cells, they are often grouped with lymphocytes. NK cells provide non-specific immunity against cells displaying foreign proteins, such as virally infected cells and certain cancer cells, by releasing cytotoxic granules.

2. Monocytes

Monocytes are the largest type of white blood cell, classified as agranulocytes, and play a crucial role in both innate immunity and in bridging innate and adaptive immune responses. They are versatile phagocytic cells and precursors to macrophages and dendritic cells.

Characteristic Features:

• Size: The largest circulating white blood cells, typically 14-20 µm in diameter, about twice the size of red blood cells.
• Nucleus: Large, often kidney-bean shaped, horseshoe-shaped, or irregularly lobed nucleus. It is less dense and typically stains pale bluish-violet.
• Cytoplasm: Abundant, cloudy, opaque, and blue-grey cytoplasm, often containing fine lilac granules (lysosomes) and infrequent vacuoles.
• Abundance: Account for about 2-8% of the total white blood cell count.
• Lifespan: Circulate in the blood for about 1-3 days before migrating into tissues.

Specific Functions:

• Phagocytosis: Monocytes are highly phagocytic. They engulf and digest bacteria, fungi, viruses, cellular debris, and dead cells directly in the bloodstream.
• Differentiation into Macrophages: Once they migrate into tissues, monocytes differentiate into macrophages. Macrophages are long-lived phagocytes that continue to clear pathogens and debris. They are crucial for maintaining tissue homeostasis and initiating tissue repair by clearing dead cells and releasing growth factors.
• Antigen Presentation: Monocytes, and their differentiated forms (macrophages and dendritic cells), act as Antigen-Presenting Cells (APCs). They process ingested pathogens and present fragments (antigens) on their cell surface to T lymphocytes, thereby activating adaptive immune responses. This is a critical link between innate and adaptive immunity.
• Cytokine Secretion: Monocytes secrete various cytokines, which are signaling molecules that modulate immune and inflammatory responses, attracting other immune cells to the site of infection or injury.

3. Neutrophils

Neutrophils are the most abundant type of white blood cell and are classified as granulocytes. They are the body's first line of defense against bacterial and fungal infections, acting as rapid responders in acute inflammation.

Characteristic Features:

• Size: Medium-sized leukocytes, typically 9-16 µm in diameter, larger than red blood cells.
• Nucleus: Characteristic multi-lobed nucleus, usually with 2 to 5 lobes connected by thin strands of chromatin. This gives them the name "polymorphonuclear leukocytes" (PMNs).
• Cytoplasm: Contains numerous fine, neutral-staining granules (hence "neutrophil") that appear pink or purple-blue. These granules contain a variety of enzymes (e.g., lysozyme, collagenase, elastase) and antimicrobial proteins.
• Abundance: The most abundant white blood cells, making up 50-70% of the circulating leukocytes.
• Lifespan: Relatively short lifespan of 6-10 hours in the bloodstream, then 1-2 days in tissues. They are continuously produced in the bone marrow.

Specific Functions:

• First Responders: Neutrophils are the primary and rapid responders to bacterial and fungal infections. They are quickly recruited to sites of inflammation or infection through chemotaxis.
• Phagocytosis: Their main function is to engulf and digest bacteria and other microorganisms through phagocytosis. After engulfing a pathogen into a phagosome, they fuse it with granules (lysosomes) to form a phagolysosome, where the pathogen is rapidly destroyed by enzymatic degradation and reactive oxygen species (respiratory burst).
• Degranulation: Neutrophils release the contents of their granules into the extracellular space to destroy pathogens and contribute to the inflammatory response.
• Neutrophil Extracellular Traps (NETs): They can also release their decondensed chromatin, loaded with antimicrobial proteins, to form web-like structures called NETs. These NETs trap and kill bacteria and fungi, preventing their spread.
• Inflammatory Response: Their activity at infection sites contributes to the signs of acute inflammation (redness, swelling, heat, pain) and the formation of pus (dead neutrophils and cellular debris).

Comparative Overview of Lymphocytes, Monocytes, and Neutrophils

Feature	Lymphocytes	Monocytes	Neutrophils
Type of WBC	Agranulocyte	Agranulocyte	Granulocyte
Abundance (% of WBC)	20-40%	2-8%	50-70%
Size	Small (6-9 µm)	Largest (14-20 µm)	Medium (9-16 µm)
Nucleus Shape	Large, spherical, dense	Kidney-shaped, horseshoe, or irregularly lobed	Multi-lobed (2-5 lobes)
Cytoplasm	Scant, clear, basophilic, no visible granules	Abundant, cloudy, blue-grey, fine lilac granules, vacuoles	Abundant, pale pink/purple, fine neutral-staining granules
Primary Role	Adaptive Immunity	Innate Immunity, Bridge to Adaptive Immunity	Innate Immunity (First Responders)
Key Functions	Antibody production (B cells), direct killing of infected cells (Cytotoxic T cells), immune regulation (Helper T cells, Regulatory T cells), non-specific killing (NK cells)	Phagocytosis, differentiation into macrophages/dendritic cells, antigen presentation, cytokine secretion, tissue repair	Phagocytosis of bacteria/fungi, degranulation, NET formation, acute inflammation
Lifespan (Circulation)	Weeks to years (memory cells); most are short-lived	1-3 days	6-10 hours