The majority of the specialized cells used in the immune system of the body are, in fact, white blood cells. There are five different types of white blood cells. Collectively they are called leukocytes. There are two main varieties of white cells in the blood. Those that do not contain granules are called lymphocytes. Lymphocytes have regular round nuclei. The other types, which have segmented nuclei, are called granulocytes. The granulocyte group includes neutrophils, basophils, and eosinophils along with a variety of other leukocytes. A healthy human has about 5,000 white blood cells per cubic millimeter, which makes up about 1% of the total blood volume in the body.
B lymphocyte cells, called bursa-dependent, are manufactured in the bone marrow, spleen and other blood-forming tissues. B cells are a main component of the humoral and cellular immune responses of the body. These cells have special receptors, which allow them to identify antigens and produce antibodies. These lymphocytes do not venture into infected cells and tissue; B cells prefer to stay in the body fluids and neutralize invaders. When they mature, they are called plasma cells and secrete antibodies.
Some B cells transform into memory cells in the last stages of an immune response. They multiply quickly and circulate through the entire body. The job of these cells is to keep the body prepared for the next struggle with antigens that have already been encountered. Memory cells give the immune system a head start on recurring infections and prevent the body from contracting certain diseases more than once.
Most of the time, B lymphocyte cells that encounter antigens will transform into antibody factories. These manufacturing plants, called plasma cells, produce antibodies to help in the fight. However, antibodies are occasionally produced in the bone marrow and strewn throughout the lymphoid tissues. They are stored in the spleen, small intestine and lymph nodes. Antibodies have very specific targets; they are specially manufactured to handle certain antigens. They bind to antigens on the surfaces of viruses and help macrophages destroy them. The production of antibodies is part of acquired immunity.