© 2003 by Taylor & Francis Group, LLC. The primary function of the immune system is to protect the body from the deleterious effects of pathogenic organisms such as bacteria, viruses, fungi, and parasites and consequently from infection. This immune defensive activity is carried out primarily by leukocytes, which include lymphocytic cells (i.e. T and B cells and natural killer cells) and phagocytic cells (i.e. macrophages and neutrophils). Leukocytes are predominately found in the lymphoid organs [i.e. bone marrow, thymus, spleen, lymph nodes, and mucosa-associated lymphoid tissues (MALT)], but are also present in other tissues, such as the liver and lungs.