Immunity

Humans and all other vertebrates react to the presence of parasites within their tissues by means of immune mechanisms of which there are two types: nonspecific, innate immunity and specific, acquired immunity. Innate immunity, with which an organism is born, involves protective factors, such as interferon, and cells, such as macrophages, granulocytes, and natural killer cells, and its action does not depend on prior exposure to a pathogen. Specific immunity is acquired during the organism’s lifetime and involves the activation of white blood cells (B and T lymphocytes), which distinguish and react to foreign substances. B lymphocytes (or B cells) operate by producing antibodies, proteins that neutralize foreign molecules (antigens), while T lymphocytes (or T cells) directly attack invaders. Many immune responses, however, involve both mechanisms.