The immune system is a complex system which provides our bodies with a means of protecting ourselves from [infection|infections]], invasive agents and damaged cells which can become cancerous. The immune response is the activation of the immune system by the presence of antigens (usually proteins) found on the surface of cells, viruses, fungi, bacteria and other substances such as chemicals and foreign particles (like a splinter). The components of the immune system include cells and molecules that recognize and destroy antigens, such as antigen-presenting cells (B cells, monocytes, macrophages and dendritic cells), polymorphonuclear leukocytes (neutrophils, eosinophils, basophils and mast cells), cytotoxic leukocytes (natural killer cells and lymphokine-activated killer cells), lymphocytes (B cells and T cells), antibodies, acute phase cell reactants, and cytokines. http://www.merck.com/mmpe/sec13/ch163/ch163b.html http://www.nlm.nih.gov/medlineplus/ency/article/000821.htm

In the blood, there are white blood cells (leukocytes) which do not have hemoglobin and molecules which mediate the immune response, such as interferon, complement proteins and cytokines. The leukocytes are produced by stem cells in the bone marrow and they mature in the thymus. Agranular leukocytes include the lymphocytes, B cells and T cells. Interferons, consider part of the non-specific immune system, are produced in a localized area by leukocytes in response to a stimulus before the other parts of the immune system are activated. Cytokines, or leukokines, are proteins produced by lymphocytes that help mediate the acquired immunity response and are found in tissues where inflammation is present. Complement proteins are part of the innate immune system. There are about 25 different complement proteins that circulate in the blood to complement the antibodies for the destruction of bacteria. http://www.cancer.gov/cancertopics/understandingcancer/immunesystem/Slide18 http://www.biology-online.org/dictionary/Leukocyte http://pathmicro.med.sc.edu/mhunt/interferon.htm