Your immune system is responsible for helping to eliminate invaders (antigens) such as infectious organisms. The key cells in your immune system are lymphocytes known as B cells and T cells, which originate in your bone marrow. After T cells further develop in your thymus, all of your immune system cells gather in your lymph nodes and spleen. Antigens (triangular shapes above) are ingested (1), partially digested (2) and then presented to helper T cells by special cells called macrophages (3). This process activates the helper T cell to release hormones (lymphokines) that help B cells develop (4). These hormones, along with recognition of further antigens (5), change the B cell into an antibody-producing plasma cell (6). The antibodies (Y shapes above) produced can be one of several types (IgG, IgM, IgA, IgE and IgD) (7). The antibody "fits" the antigen much like a lock fits a key. The antigen is thus rendered harmless. The helper T cells also aid in development of cytotoxic T cells (8), which can kill antigens directly; memory T cells are produced (9) so that re-exposure to the same antigen will provide a more rapid and effective response (10).