When the immune system detects the antigens on the surfaces of invading organisms, either a humoral or cell-mediated immune response is triggered. Humoral immune responses resist pathogens, such as bacteria and toxins, that act outside cells and also prevent viruses from entering cells. During such a response, macrophages ingest some of the antigen and stick it to class II MHC molecules. These molecules then expose the antigen to helper T cells which bind to the antigen and secrete interleukins. The interleukins stimulate any B cells bound to antigens on the invaders' surfaces to divide and make antibodies. Finally, the antibodies, or immunoglobulins, bind themselves to the antigens and neutralize or opsonize the antigens, or trigger the complement system. B cells can secrete any of the five major types of antibodies, depending on which type of interleukin they receive.
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During a cell-mediated immune response, cells, known as white blood cells, become active. There are a number of specialized white blood cells, but the two most generic types are macrophages and lymphocytes. Cell-mediated immune responses resist pathogens that use cells to reproduce, such as viruses, and cancerous cells.
In the first step of a cell-mediated immune response, a few macrophages go through the process of phagocytosis and expose helper T cells to samples of the antigen. Next, the helper T cells produce molecules called interleukins that stimulate any killer T cells bound to the antigens on infected cells. Finally, the activated T cells seek out and destroy any other cells in the body displaying the same antigens.