Acquired (Adaptive) Immunity

Where innate immunity treats every threat in roughly the same way, acquired immunity (also called adaptive or specific immunity) is tailor-made for each individual pathogen the body meets, and — most importantly — it remembers. This immunity is mediated by two kinds of white blood cells called lymphocytes: B-lymphocytes and T-lymphocytes, both of which originate in the bone marrow. B-lymphocytes mature in the bone marrow itself, while T-lymphocytes migrate to and mature in the thymus gland.

Acquired immunity has two closely linked arms:

Humoral immunity, mediated by B-lymphocytes, is the arm responsible for antibody production. When a B-cell encounters a matching antigen, it multiplies and differentiates into plasma cells that secrete large quantities of antibody specific to that antigen, as introduced in section 7.1.

Cell-mediated immunity, mediated by T-lymphocytes, does not rely mainly on secreted antibodies but instead uses cells to directly attack infected or abnormal cells. Helper T-cells coordinate the overall immune response and assist B-cells in making antibodies; cytotoxic (killer) T-cells directly destroy virus-infected cells and tumour cells; and regulatory T-cells help bring the immune response back under control once the threat has been dealt with.

Two features make acquired immunity distinct from the innate system described in section 7.2:

Specificity — each B- or T-lymphocyte response is directed against one particular antigen, not against pathogens in general. …