Phagocytes and Pinocytosis

Endocytosis

process by which cells take in materials from outside the cell

Phagocytosis

“cell eating”; a specific form of endocytosis where cells bind, internalize, and destroy foreign substances

Professional phagocytes

neutrophils, macrophages, and dendritic cells specialized for phagocytosis

Neutrophils (PMNs)

polymorphonuclear leukocytes that rapidly respond to infection and perform phagocytosis

Macrophages

tissue‑resident phagocytes specialized by location

Dendritic cells

antigen‑presenting phagocytes found in tissues

Oxidative killing

microbial killing using reactive nitrogen intermediates (NO) and reactive oxygen intermediates (superoxide via NADPH oxidase)

Non‑oxidative killing

microbial killing via phagosome‑lysosome fusion, proteolytic enzymes, and cationic antimicrobial peptides

Antigen presentation

process where macrophages and dendritic cells present degraded antigen fragments to adaptive immune cells

Inflammatory mediators

molecules produced by phagocytes to promote inflammation

Recognition and adherence

first step of phagocytosis where receptors bind foreign particles

Non‑opsonic receptors

PRRs that recognize PAMPs and directly induce phagocytosis

Opsonin

extracellular protein (e.g. , antibody, C3b) that labels foreign particles for phagocytosis

Opsonic receptors

receptors that bind opsonins to enhance phagocytosis

Fcγ receptor (FcγR)

receptor that binds the Fc region of IgG antibodies

Complement receptors

receptors that bind activated complement molecules on microbes

Engulfment

step where receptor‑ligand binding triggers pseudopod formation and phagocytic cup development

Phagosome

membrane‑bound vesicle containing the engulfed particle

Phagosome maturation

sequential transformation of the phagosome through fusion with endocytic vesicles

Early phagosome

Rab5‑positive phagosome that fuses with early endosomes via EEA1

Late phagosome

Rab7‑positive phagosome that fuses with late endosomes

Phagolysosome

acidic, microbicidal organelle formed by fusion of phagosome and lysosome

Pinocytosis

“cell drinking”; uptake of extracellular fluid and dissolved solutes

Sentinel cells

macrophages that continuously sample the extracellular environment via pinocytosis

Immune surveillance

monitoring of the environment by macrophages/DCs through pinocytosis

Virus entry via pinocytosis

mechanism where viruses (e.g., SARS‑CoV‑2, Ebola) hijack pinocytosis to enter cells

Neutrophilia

disorder of neutrophil number; abnormally high neutrophil count in blood; leads to uncontrollable immune response; indicator of acute infection

Primary neutrophilia

increased neutrophil production due to bone marrow dysregulation

Secondary neutrophilia

increased neutrophils due to infection or inflammation

Neutropenia

disorder of neutrophil number; abnormally low neutrophil count in blood; leads to increased susceptibility to infection

Primary neutropenia

low neutrophils due to bone marrow failure or infiltration

Secondary neutropenia

low neutrophils due to infection, drugs, or radiation