If foreign invaders succeed in getting past your skin barriers and manage to reach body tissues, they are usually recognized, ingested, and killed by phagocytes strategically positioned throughout the body. Macrophages and neutrophils are the main phagocytes involved, with macrophages as the first line of defense. Monocytes stop circulating in the blood and mature into specialized macrophages that migrate into the tissues of the body and prepare for invasion. Large numbers of mature macrophages reside in connective tissue, along the digestive tract, in the lungs, in the spleen, and even along certain blood vessels in the liver, where they are known as Kupffer cells.

Neutrophils are short-lived immune cells that remain circulating in the blood. When tissue-based macrophages encounter an invader, neutrophils soon reinforce their immune response by coming to the site of infection in large numbers.

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