The immune system cell types :) What is the immune system? The body's innate defense against disease is natural immunity. The immune system consists of white blood cells, cell products, and other substances.Lymphocytes include:-T cells (T for thymus, where they mature)-B cells (B for bone marrow, where they originate)-Natural killer (NK) cells (less abundant)Other white blood cells include monocytes-macrophages and granulocytes (neutrophils, eosinophils, and basophils). They are phagocytic or "cell-eating." The fabric of the immune system is lymphoid tissue, which includes bone marrow, the thymus (a gland beneath the breastbone), lymph nodes, spleen, tonsils, gut and other mucosal areas.Complement is a soluble protein secretion in blood that destroys pathogens when it is activated.The cells of the immune system can be identified by their microscopic appearance and by surface CD markers, of which there are nearly two hundred.How does the immune system know when to respond?The immune system goes into a "search-out and destroy" mode when it "recognizes" foreign antigens. An antigen is any of an enormous range of substances that can be bound by an antibody and induce some kind of immune response.The immune system's first encounter with an antigen is the primary response. Re-encounter with the same antigen causes a more powerful and rapid secondary response. This acquired immunity involves lymphocytes.Activated lymphocytes divide and produce chemokines, the chemical signals between cells of the immune system. Some lymphocytes may never have been exposed to the antigen; others "remember" it. These cells are called "naïve" (unprimed) cells and "memory" (primed) cells.The main immune defenses against tumors are T cells, natural killer cells, and macrophages. Together with B cells and complement, they also play a role against infections.
