Learning Objective

Describe the morphology, function, and differentiation of monocytes, and explain their role in innate immunity and antigen presentation.

Monocytes

Monocytes are circulating mononuclear leukocytes found in peripheral blood that serve as precursors to tissue macrophages and dendritic cells. They are part of the innate immune system and play a key role in phagocytosis and antigen presentation.

Morphology

• Large cells (the largest normal WBC in peripheral blood)
• Kidney-shaped (horseshoe-shaped) nucleus
• Abundant pale blue-gray cytoplasm with a characteristic “frosted glass” appearance
• May contain fine azurophilic granules

Differentiation

After migrating into tissues, monocytes differentiate into:

• Macrophages → specialized for phagocytosis and cytokine production
• Dendritic cells → professional antigen-presenting cells (APCs)

Function

• Phagocytose pathogens and cellular debris
• Present antigen to T lymphocytes (via MHC II)
• Produce inflammatory cytokines (e.g., IL-1, TNF-α)
• Contribute to chronic inflammation and granuloma formation

Clinical Relevance

• Monocytosis may be seen in chronic infections (e.g., tuberculosis), autoimmune disease, and certain leukemias.
• Tissue macrophages have specialized names depending on location (e.g., Kupffer cells in the liver, microglia in the CNS).

Activity