The immune system amounts to a concerted set of cells with specialized roles, some of which overlap.
White blood cells (leukocytes) are a mainstay of the immune system. The number of these cells in a human body runs to the billions.
All types of leukocytes are made in bone marrow, derived from stem cells. Stem cells – found in all multicellular organism – can differentiate into different bodily cell types.
There are distinctive types of white blood cells. 50% to 60% all leukocytes are granulocytes.
Granulocytes contain granules in their cytoplasm, hence the name. There are 3 classes of granulocyte: neutrophils, eosinophils, and basophils. The granules in granulocytes carry distinct chemicals based upon cell type. Granulocyte chemicals facilitate many functions, including purging pathogens.
Neutrophils are first-responder phagocytes that attempt to engulf invaders. Phagocytes are cells that ingest foreign particles and cellular waste. Neutrophils are recruited to the site of an injury within minutes. The predominant cells of pus, neutrophils are the hallmark of acute inflammation.
Eosinophils combat infections and parasites.
Basophils release chemicals and enzymes that contribute to inflammation, speeding blood flow to an infected site.
The other white blood cells are agranulocytes: non-granular cells that comprise over a 1/3rd of the leukocytes circulating in the bloodstream. There are 2 types of agranulocyte: lymphocytes and monocytes.
Lymphocytes may be large or small. Natural killer (NK) cells, players in innate immunity, are relatively big. NK’s smaller cousins work in the department of acquired immunity: B and T cells. B cells produce antibodies: tags that stick to pathogens, marking them for destruction. Helper and killer T cells respectively coordinate and attack viral infections.
Monocytes are immune system janitors. During phagocytosis monocytes and neutrophils vacuum up after infection, clearing away foreign matter. Monocytes also present pathogen pieces to T cells so the invaders can be remembered. Monocytes that migrate from the bloodstream into tissue are called macrophages.
Monocytes and macrophages are phagocytes, which make up around 7% of all leukocytes. Phagocytes play important roles in fighting infections and maintaining health, including longevity.