white blood cells

Meaning: Leukocytes are nucleated blood cells without hemoglobin, and they are far less numerous than red blood cells. They derive from hematopoietic stem cells in bone marrow. The myeloid lineage gives neutrophils, eosinophils, basophils and monocytes (which become macrophages); the lymphoid lineage gives B cells, T cells and natural killer cells. Neutrophils and macrophages phagocytose pathogens, eosinophils respond to parasites, and basophils release histamine. B cells become plasma cells that secrete antibodies. T cells mature in the thymus as helper (CD4) or cytotoxic (CD8) cells.

On the MCAT: Passages describe an infection, vaccine or immune deficiency and ask which cell is involved. Expect these points: memory B and T cells make a second exposure faster and stronger than the first; losing helper T cells weakens both antibody and cell-mediated responses; cytotoxic T cells recognize antigen on MHC I, while helper T cells recognize it on MHC II of antigen-presenting cells. Leukocytes can leave capillaries by squeezing between endothelial cells (diapedesis), and counts often rise during infection.

Common confusion: Students credit T cells with making antibodies. Antibodies come from plasma cells, which derive from B cells; T cells coordinate the response or kill infected cells. Also, innate cells respond to broad pathogen features, while lymphocytes respond to specific antigens.