leukocytes

Meaning: Leukocytes arise from hematopoietic stem cells in bone marrow, travel in blood and lymph, and can squeeze out of capillaries (diapedesis) into tissues. Unlike mature red blood cells, they have nuclei. The myeloid lineage gives neutrophils, eosinophils, basophils and monocytes (which become macrophages in tissue). The lymphoid lineage gives B cells, T cells and natural killer cells. Granulocytes have prominent cytoplasmic granules; lymphocytes and monocytes do not.

On the MCAT: Expect questions that match cell type to role. Neutrophils and macrophages phagocytose (innate, nonspecific). Eosinophils respond to parasites, and basophils release histamine. B cells make antibodies, helper T cells (CD4) coordinate the response, and cytotoxic T cells (CD8) kill infected cells (adaptive, specific, with memory). T cells mature in the thymus. A passage may give a differential count and ask which cell type should rise in a bacterial infection, a parasitic infection or an allergy.

Common confusion: Students treat every leukocyte as a phagocyte, or as part of adaptive immunity. Many leukocytes are not phagocytes, and antigen-specific adaptive responses are carried out by lymphocytes. Platelets are cell fragments, not leukocytes.