Phagocytosis is a critical process in the body's innate immunity, where macrophages internalize particles through an actin-dependent mechanism. While previous studies have focused on spherical targets, this research reveals that particle shape, not size, is the dominant factor in determining whether macrophages internalize targets or simply spread on them. The local shape of the particle at the point of initial contact dictates the complexity of the actin structure required for internalization. If the actin structure cannot be formed, macrophages spread on the particle rather than internalize it. Particle size primarily affects the completion of phagocytosis when the target volume exceeds the macrophage volume. In this study, alveolar macrophages were used to phagocytose polystyrene (PS) particles of various sizes and shapes. The results showed that the orientation of attachment significantly influenced phagocytosis. For example, macrophages attached to the major axis of elliptical disks (EDs) internalized the particles quickly, while those attached to the minor axis or flat side only spread on the particle. This difference was attributed to the local shape of the particle, which determined the actin structure needed for internalization. The angle Ω, defined as the angle between the membrane normal at the point of initial contact and a vector representing the mean direction of the particle's surface, was used to quantify the effect of shape on phagocytosis. Particles with Ω values below 45° were internalized, while those with Ω values above 45° resulted in spreading. This threshold of 45° marked a critical transition between phagocytosis and spreading. The study also showed that the rate of phagocytosis decreased with increasing Ω, and internalization velocity dropped to zero at Ω ≈ 45°. The findings highlight the importance of particle geometry in phagocytosis, revealing that macrophages respond to local shape rather than overall size. This has implications for understanding phagocytosis of natural targets and for designing drug delivery systems that can either promote or avoid phagocytosis. The study provides a framework for analyzing the complex interplay between shape and size in phagocytosis, emphasizing the role of physical environment in cell behavior.Phagocytosis is a critical process in the body's innate immunity, where macrophages internalize particles through an actin-dependent mechanism. While previous studies have focused on spherical targets, this research reveals that particle shape, not size, is the dominant factor in determining whether macrophages internalize targets or simply spread on them. The local shape of the particle at the point of initial contact dictates the complexity of the actin structure required for internalization. If the actin structure cannot be formed, macrophages spread on the particle rather than internalize it. Particle size primarily affects the completion of phagocytosis when the target volume exceeds the macrophage volume. In this study, alveolar macrophages were used to phagocytose polystyrene (PS) particles of various sizes and shapes. The results showed that the orientation of attachment significantly influenced phagocytosis. For example, macrophages attached to the major axis of elliptical disks (EDs) internalized the particles quickly, while those attached to the minor axis or flat side only spread on the particle. This difference was attributed to the local shape of the particle, which determined the actin structure needed for internalization. The angle Ω, defined as the angle between the membrane normal at the point of initial contact and a vector representing the mean direction of the particle's surface, was used to quantify the effect of shape on phagocytosis. Particles with Ω values below 45° were internalized, while those with Ω values above 45° resulted in spreading. This threshold of 45° marked a critical transition between phagocytosis and spreading. The study also showed that the rate of phagocytosis decreased with increasing Ω, and internalization velocity dropped to zero at Ω ≈ 45°. The findings highlight the importance of particle geometry in phagocytosis, revealing that macrophages respond to local shape rather than overall size. This has implications for understanding phagocytosis of natural targets and for designing drug delivery systems that can either promote or avoid phagocytosis. The study provides a framework for analyzing the complex interplay between shape and size in phagocytosis, emphasizing the role of physical environment in cell behavior.