Lymphatic (lymphoid) tissue is a kind of connective tissue. It consists of the following types of cells: Lymphocytes are white blood cells ( leukocytes) that provide an immune response that attacks specific kinds of nonself cells and foreign substances (antigens). There are several major classes of lymphocytes: T cells (T lymphocytes) originate in the bone marrow but mature in the thymus gland. T cells attack self cells that have been invaded by pathogens, abnormal self cells (such as cancerous cells), or nonself cells (such as those that might be introduced in an organ transplant). B cells (B lymphocytes) originate and mature in the bone marrow. When B cells encounter an antigen (a toxin, virus, or bacterium), they produce plasma cells and memory cells. Plasma cells release antibodies that bind to the antigen and inactivate it. Memory cells circulate in the lymph and blood with the capacity to produce additional antigens for future encounters with the same antigen. Macrophages are enlarged monocytes (white blood cells) that engulf microbes and cellular debris. Reticular cells and their reticular fibers made from collagen and glycoproteins provide a network within which the lymphocytes and other cells reside.