Phagocytosis, a key component of the body's innate immunity, involves macrophages internalizing targets through an actin-dependent process. Targets vary widely in shape and size, including pathogens, senescent cells, and airborne particles. Despite progress in understanding phagocytosis, the role of target geometry has remained unclear due to reliance on spherical targets in previous studies. This study used alveolar macrophages and polystyrene particles of various shapes and sizes to investigate the role of target geometry in phagocytosis. The findings reveal that particle shape, not size, plays a dominant role in determining whether macrophages initiate phagocytosis or simply spread on the particle. The local shape at the point of initial contact dictates the complexity of the actin structure required for internalization. If this structure is not formed, macrophages spread on the particle without internalizing it. Particle size primarily affects the completion of phagocytosis when the target volume exceeds the cell volume. The study used geometrically anisotropic polystyrene particles, including spheres, ellipsoids, elliptical disks, rectangular disks, and UFOs. Results showed that macrophages internalized particles when attached along the major axis but not when attached along the minor axis or flat side. Scanning electron microscopy and actin staining confirmed that the formation of an actin cup or ring is crucial for internalization. The angle Ω, defined as the angle between the membrane normal and a vector representing the mean direction of tangents, was found to be a key parameter in determining phagocytosis. Particles with Ω ≤ 45° were internalized, while those with Ω > 45° resulted in spreading. The phase diagram of phagocytosis shows three regions: successful phagocytosis (Ω ≤ 45°, V* ≤ 1), attempted phagocytosis (Ω ≤ 45°, V* > 1), and spreading (Ω > 45°). The study highlights the importance of target geometry in phagocytosis and its implications for drug delivery and cell behavior.Phagocytosis, a key component of the body's innate immunity, involves macrophages internalizing targets through an actin-dependent process. Targets vary widely in shape and size, including pathogens, senescent cells, and airborne particles. Despite progress in understanding phagocytosis, the role of target geometry has remained unclear due to reliance on spherical targets in previous studies. This study used alveolar macrophages and polystyrene particles of various shapes and sizes to investigate the role of target geometry in phagocytosis. The findings reveal that particle shape, not size, plays a dominant role in determining whether macrophages initiate phagocytosis or simply spread on the particle. The local shape at the point of initial contact dictates the complexity of the actin structure required for internalization. If this structure is not formed, macrophages spread on the particle without internalizing it. Particle size primarily affects the completion of phagocytosis when the target volume exceeds the cell volume. The study used geometrically anisotropic polystyrene particles, including spheres, ellipsoids, elliptical disks, rectangular disks, and UFOs. Results showed that macrophages internalized particles when attached along the major axis but not when attached along the minor axis or flat side. Scanning electron microscopy and actin staining confirmed that the formation of an actin cup or ring is crucial for internalization. The angle Ω, defined as the angle between the membrane normal and a vector representing the mean direction of tangents, was found to be a key parameter in determining phagocytosis. Particles with Ω ≤ 45° were internalized, while those with Ω > 45° resulted in spreading. The phase diagram of phagocytosis shows three regions: successful phagocytosis (Ω ≤ 45°, V* ≤ 1), attempted phagocytosis (Ω ≤ 45°, V* > 1), and spreading (Ω > 45°). The study highlights the importance of target geometry in phagocytosis and its implications for drug delivery and cell behavior.