Cells destined to become immune cells, like all blood cells, arise in your body's bone marrow from stem cells. Some develop into myeloid progenitor cells while others become lymphoid progenitor cells.

The myeloid progenitors develop into the cells that respond early and nonspecifically to infection. Neutrophils engulf bacteria upon contact and send out warning signals. Monocytes turn into macrophages in body tissues and gobble up foreign invaders. Granule-containing cells such as eosinophils attack parasites, while basophils release granules containing histamine and other allergy-related molecules.

Lymphoid precursors develop into the small white blood cells called lymphocytes. Lymphocytes respond later in infection. They mount a more specifically tailored attack after antigen-presenting cells such as dendritic cells (or macrophages) display their catch in the form of antigen fragments. The B cell turns into a plasma cell that produces and releases into the bloodstream thousands of specific antibodies. The T cells coordinate the entire immune response and eliminate the viruses hiding in infected cells.

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