Immunity and the Immune System

Immunity is the overall ability of the body to resist and defend itself against disease-causing organisms (pathogens) and other foreign substances (antigens). It is carried out by the immune system — the coordinated network of organs, tissues, cells and molecules (including the bone marrow, thymus, lymph nodes, spleen, circulating lymphocytes and phagocytes, and secreted antibodies) that together detect and eliminate anything the body recognises as foreign.

Immunity is usually described as having two complementary arms. Non-specific (innate) immunity is present in the body from birth, acts immediately, and does not depend on having met the particular invader before. Specific (acquired) immunity develops only after exposure to a particular antigen, is highly specific to that antigen, and — crucially — leaves behind a memory that allows a faster, stronger response the next time the same antigen is encountered. The rest of this chapter looks first at the non-specific defences and then at the cellular and antibody-based machinery of the specific immune response.