Differentiation and subpopulations
The process of monocyte maturation into a macrophage is regulated by the tissue microenvironment and macrophage colony-stimulating factor (M-CSF). During this process, the cell increases in size, reorganizes its cytoskeleton, and alters its set of membrane receptors (for example, it loses CD13 and myeloperoxidase, but increases the expression of CD16 and transglutaminase).
In the bloodstream, monocytes are heterogeneous and divided into two main pools:
- Classical (inflammatory): have the CD14$^{hi}$CD16$^{-}$ phenotype. These are large cells with high phagocytic activity. They are ready to migrate to the focus of inflammation, carrying numerous $\beta_2$-integrins and chemokine receptors. They are precursors of inflammatory macrophages.
- Non-classical (intermediate): phenotype CD14$^{+}$CD16$^{+}$. They are poorer phagocytes, but excellent at presenting antigens to T lymphocytes. They are considered precursors of resident (tissue) macrophages.
Receptor apparatus
Macrophages possess a colossal arsenal of receptors for recognizing pathogen-associated molecular patterns (PAMPs) and interacting with other cells:
- Toll-like receptors (TLRs). Represented as broadly as possible. Surface receptors (TLR-1, 2, 4, 5, 6, 11) scan for threats externally, while intracellular receptors (TLR-3, 7, 8, 9) sit on granule membranes and recognize foreign nucleic acids.
- CD14 molecule. Works in tandem with TLR-4. It binds bacterial lipopolysaccharide (LPS) and transfers it to TLR-4 to trigger activation.
- Lectin receptors (e.g., mannose receptor). Recognize free sugar residues. In healthy body cells, these are hidden under sialic acid, so the macrophage attacks only pathogens or old cells destined for destruction.
- Fc receptors. Necessary for capturing antibody-coated (opsonized) targets. Interestingly, monocytes are the only cells that spontaneously carry the Fc$\gamma$RI receptor (CD64), which is capable of binding even free IgG.
- Scavenger receptors. For example, CD36 binds to collagen, which also serves as a signal for phagocytosis.
Functions: phagocytosis, secretion, and presentation
Although the primary task of macrophages is phagocytosis, their physiological role hardly overlaps with that of neutrophils. Neutrophils act quickly and aggressively at the earliest stages. Macrophages, on the other hand, phagocytize more slowly but for longer periods, and take on the role of inflammation "directors" through potent secretion.
Unlike granulocytes, macrophages predominantly use the classical mechanism of secretion, which depends on the Golgi apparatus rather than simple degranulation. They release pro-inflammatory cytokines (IL-1, IL-6, TNF), interferons, growth factors, and complement components.
In addition, macrophages are professional antigen-presenting cells. They digest the pathogen and display its fragments on MHC-II molecules for T lymphocytes. For full immune activation, the macrophage also uses costimulatory molecules (CD80 appears upon activation, while CD86 is constitutively present). Clinical nuance: some macrophages carry the CD4 receptor, making them vulnerable to infection by the human immunodeficiency virus (HIV).
Tissue varieties
During routine migration into tissues, monocytes transform into resident macrophages, which live for years and maintain local homeostasis. Depending on the organ, they receive different names and a unique set of markers:
- Microglia — in the central nervous system.
- Kupffer cells — in the liver.
- Alveolar macrophages — in the lungs.
- Red and white pulp macrophages — in the spleen.
In the event of emergency migration driven by infection, inflammatory macrophages are formed. Their lifespan is counted in weeks, and their main task is the destruction of pathogens and subsequent healing of damaged tissues.
Briefly: answers to topical questions
Which cytokines and biologically active substances do macrophages secrete upon activation?
Upon activation, macrophages secrete a wide range of cytokines and biologically active substances.
- Cytokines — pro-inflammatory (TNF$\alpha$, IL-1, IL-6, IL-12, IL-18, IL-23, IL-27), suppressive (IL-10, TGF$\beta$), and homeostatic (IL-15).
- Chemokines — pro-inflammatory (CXCL8, CCL5), macrophage inflammatory, and chemotactic proteins.
- Interferons — IFN$\alpha$, IFN$\beta$, IFN$\gamma$.
- Colony-stimulating factors — GM-CSF, G-CSF, M-CSF.
- Growth factors — FGF, PDGF, VEGF.
- Eicosanoids — prostaglandins, leukotrienes.
- Reactive oxygen and nitrogen species — $O_2^-$, $H_2O_2$, $OH^*$, NO, $OO^*NO$.
- Other substances — complement components, enzymes, matrix proteins, blood clotting factors, and hormones.
What are resident tissue macrophages called in various organs?
Resident tissue macrophages have specific names depending on their localization in organs.
- Histiocytes — in connective tissue and the lymphoid system.
- Kupffer cells (stellate endothelial cells) — in the liver.
- Osteoclasts — in bone tissue.
- Microglia — in the central nervous system (brain).
- Alveolar macrophages — in the lungs.
- Mesangial cells — in renal glomeruli.
- Synovial cells (type A) — in joints.
- Red and white pulp macrophages — in the spleen.
- Cavity macrophages — pleural and peritoneal.
What stages of phagocytosis do macrophages sequentially perform?
Macrophages sequentially carry out eight stages of phagocytosis.
- Chemotaxis — directed movement of the phagocyte toward the object and cell polarization.
- Adhesion — physical interaction and attachment of the phagocyte to the object after its recognition.
- Membrane activation — preparation of the cell to engulf the target.
- Engulfment — wrapping of the object by cytoskeletal microfilaments.
- Phagosome formation — closure of the membrane around the ingested particle.
- Phagolysosome fusion — fusion of the phagosome and lysosome.
- Killing and digestion — intracellular digestion and destruction of the pathogen.
- Release of degradation products — degranulation and discharge of residues into the extracellular environment.
Which macrophage receptors belong to the scavenger receptor group and what do they recognize?
Macrophage scavenger receptors include receptors whose binding to ligands serves as a signal for phagocytosis.
- MSR (Macrophage scavenger receptor, CD36) — an example of a scavenger receptor; exhibits affinity for collagen.
- In atherosclerosis, macrophages capture oxidized LDL via scavenger receptors, which is associated with uncontrolled cholesterol accumulation and foam cell formation.
To avoid confusing markers: CD13 is an "adolescent" molecule (present only on monocytes and disappears in macrophages), while CD16 is an "adult" sign (increases during maturation into a macrophage).
Class and exam questions
What is the main difference between macrophages and neutrophils?
Neutrophils provide early, intense, but short-lived phagocytosis. Macrophages act prolonged and, in addition to phagocytosis, regulate inflammation via classical cytokine secretion.
Why do macrophages not attack the body's own healthy cells?
Macrophage lectin receptors search for free carbohydrate residues. On healthy cells, these are securely "hidden" (shielded) by sialic acid.
Why do monocytes and macrophages need adhesion molecules (integrins)?
They are necessary for interaction with the extracellular matrix (during migration from the vessel into tissue) and for contact with other cells during the immune response.
What else to study on this topic
- Cytokine profile: pro-inflammatory, homeostatic, and suppressive factors
- Comparative characteristics of dendritic cell subpopulations
- Oxygen-dependent and independent bactericidal mechanisms
- Mechanism of interaction between CD14, TLR-4, and bacterial LPS
- Role of microtubules and the Golgi apparatus in the secretory function of phagocytes
Master the entire topic in the "Immunology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Macrophages and Monocytes" and 723 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 23 video lessons on the subject
Other topics in the "Innate Immunity" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.