Monocytes and Macrophages

Morphological Characteristics of Monocytes

In peripheral blood, monocytes stand out due to their size—they are the largest cells (18 to 20 µm), appearing in smears as twice the size of a standard erythrocyte. Under microscopy, they must often be differentiated from lymphocytes. This differential diagnosis relies on two main criteria:

• Nuclear structure. Monocytes feature a large, polymorphic nucleus, most commonly kidney-shaped, horseshoe-shaped, or lobulated. Chromatin is distributed loosely, giving the nucleus a relatively pale appearance. In contrast, lymphocytes have regular, round, and dark (hyperchromic) nuclei.
• Cytoplasmic features. Monocytes possess a wide, pale rim of cytoplasm occupying a substantial portion of the cell. Fine azurophilic granules representing lysosomes are often visible in the perinuclear zone. Lymphocytes, conversely, have a narrow cytoplasmic rim and usually completely lack granules.

Transformation of a Monocyte into a Macrophage

While in the bloodstream, monocytes act as circulating precursors. Their true functional purpose is revealed after migration into tissues, where final differentiation into macrophages occurs.

This maturation process is accompanied by three fundamental cellular changes:

1. The cell size increases significantly.
2. The organelle apparatus develops: active accumulation of lysosomes and expansion of protein-synthesizing components take place.
3. The plasma membrane is modified: new specific receptors required for antigen capture appear on the surface.

Mononuclear Phagocyte System

All cells originating from blood monocytes capable of active phagocytosis are united into a single macrophage system. Based on mobility, they are divided into free (wandering, migrating between blood and tissues) and fixed (resident) cells permanently located in specific organs.

Specialized tissue macrophages include:

• Typical macrophages (histiocytes) — reside in loose connective tissue.
• Osteoclasts — resorb matrix in bone tissue.
• Microglial cells — protect nervous tissue (in the CNS).
• "Nurse" cells — part of erythroblastic islands in the red bone marrow.
• Interdigitating and dendritic cells — present antigens in lymphoid organs.
• Kupffer cells (stellate macrophages) — localized in the liver.
• Langerhans cells — present in the epithelium of the airways and skin.
• Mesangial cells — located in renal corpuscles.
• Decidual cells — located in the maternal part of the placenta.

Three Main Functions of Macrophages

Typical macrophages are key participants in inflammatory and immune responses, fulfilling three strictly defined tasks.

A. Phagocytosis This is mediated by receptors on the macrophage membrane for the $F_c$ fragment of immunoglobulins (Ig). If a bacterium is pre-coated with antibodies (opsonized), the macrophage easily binds and engulfs it — this is receptor-mediated phagocytosis. The cell is also capable of non-receptor-mediated phagocytosis (clearing cellular debris). The active engulfment process, similar to neutrophils, is accompanied by an oxidative "respiratory burst".

B. Antigen Presentation After capture, the foreign agent is digested inside a phagolysosome. The macrophage extracts specific antigenic determinants and displays them on its membrane surface in complex with proteins. The main goal of this mechanism is to present the antigen to T lymphocytes to trigger an immune response. Note: macrophages are not truly immunocompetent cells; they do not form clones or possess specificity, reacting to any threat stereotypically.

C. Secretory Function These cells secrete a spectrum of biologically active substances:

• Immune regulators: interleukins that activate T- and B-lymphocyte pools.
• Protective factors: interferon (to combat viruses), lysozyme (to destroy bacteria), and cytolytic factors (against various eukaryotic cells).

Pathological Forms of Macrophages

The functions of specialized macrophages are fundamentally similar to typical ones, though either phagocytosis or antigen presentation often predominates. When overloaded with material that the cell physically cannot digest, pathological morphological forms arise.

These forms include:

• "Dust" cells — alveolar macrophages whose cytoplasm is totally filled with indigestible inorganic dust particles.
• "Heart failure cells" — macrophages critically overloaded with an iron-containing pigment (hemosiderin). They form in lung tissue during prolonged chronic congestion, when macrophages are forced to phagocytose erythrocytes that have escaped blood vessels.