Immune system cells are white blood cells, or leukocytes. They can be divided into two main groups, known as lymphocytes and phagocytes. Lymphocytes include what are called B cells and T cells, all of which circulate in the blood and in the lymphatic system, where they are stored. There are many different types of lymphocytes and each has receptors which match a particular harmful substance, or antigen. Immune system cells known as phagocytes are able to consume not only potentially harmful pathogens, such as viruses and bacteria, but also damaged or dead cells from the body.
The lymphatic system is part of the immune system and consists of a network of fluid-filled channels, together with organs such as the spleen, which helps to filter the blood and fight infections. T cells and B cells both originate in the bone marrow. B cells pass into the blood stream, while T cells move to the thymus gland underneath the breast bone, where they mature. Both types of immune system cells collect in the tissues of the lymphatic system, which include the spleen and lymph nodes, or glands.
T cells are involved in the immune response in two different ways, as they can be helper cells or killer cells. The helper T cells make proteins that activate B cells and killer T cells. Killer T cells attack and destroy cancer cells and cells that have been infected. They recognize infected cells from small amounts of antigen on the cell surface, which match the receptors on the killer T cell.
B cells become fully activated after finding and attaching to their specific antigens. Then they divide to form antibody-producing cells and memory cells, which help fight off a threat more quickly if it occurs again. Antibodies are proteins that attach to antigens, inactivating them and making it easier for phagocytes to consume them. Phagocytes consist of three main groups of immune system cells, known as macrophages, granulocytes and dendritic cells.
Macrophages are large, relatively long-lived cells which mop up bacteria, dead white cells and foreign particles such as smoke. Granulocytes are important in the early stages of disease protection, when they attack harmful organisms in large numbers and consume them. Dendritic cells also contribute to immunity by eating microbes. All of the immune system cells work together with a group of proteins, known as the complement system, to defend the body from infection. The complement system can directly destroy antigens, cause inflammation, and coat antigens so that they are more easily consumed by phagocytes.