Neutrophils are the most abundant type of leukocyte in the blood. Unlike other cells, the neutrophil does not divide once it enters the tissue. This is because it needs to reserve energy to perform as the first responder to disease and injury. When a neutrophil ingests a particle, it dies; but if it is not used, it won't live more than four to 10 hours.
Macrophages are very large phagocytic cells that ingest particles found in the fluids and air that pass through organs. Large amounts of macrophages are found in the liver, lung, spleen and blood -- all organs or tissues that are vulnerable to invasion from foreign particles.
Leukocytes are a category of immune cells, and phagocytes are one of their subgroups. However, leukocytes also include lymphocytes. Lymphocytes are intelligent immune cells that remember previous foreign particles. When an invader is recognized, the lymphocytes send the neutrophils to do their work.
Phagocytosis is the process a phagocyte undergoes to consume a foreign particle in the body. Phagocytes attach to a foreign particle before they can consume it. Some particles have an exterior that makes it difficult for the phagocyte to attach. If this is the case, then proteins from the blood help out by making a thin wall around the particle. This isn't always successful, because encapsulated particles are difficult for a phagocyte to engulf. So sometimes they are pushed around in the blood and not ingested, unless the phagocyte can stabilize it against a solid surface.