文档介绍：Immunity Chapter 35 The Immune System An animal must defend itself from the many dangerous pathogens it may encounter in the environment. The Immune System A host is susceptible to a parasite if it can ’ t eliminate a parasite before it es established. The parasite is infective . The host is resistant if it is able to prevent establishment of the parasite. The parasite is noninfective . The Immune System Immune cells, red blood cells, and other white blood cells are derived from multipotential stem cells in the bone marrow. The Immune System Lymphocytes arise from stem cells in the bone marrow. Newly formed lymphocytes are all alike, but they later develop into B cells or T cells, depending on where they continue their maturation. The Immune System As B and T cells are maturing in the bone marrow and thymus, their antigen receptors are tested for possible self- reactivity. Lymphocytes bearing receptors for antigens already present in the body are destroyed by apoptosis or rendered nonfunctional. The Immune System Two major kinds of immunity have evolved that counter these invaders: Innate immunity Acquired immunity Innate Immunity Innate immunity is present before any exposure to pathogens and is effective from the time of birth. Involves nonspecific responses to pathogens. Acquired Immunity Acquired immunity develops only after exposure to inducing agents such as microbes, toxins, or other foreign substances. Involves a very specific response to pathogens. A Summary of Innate and Acquired Immunity