Vaccines may contain either living or killed organisms or purified antigens from these organisms. Vaccines containing living organisms tend to trigger the best protective responses. Killed organisms or purified antigens may be less immunogenic than living ones. Because they are unable to grow and spread in the host, they are less likely to optimally stimulate the immune system. Living viruses from vaccines, for example, infect host cells and grow. The infected cells then process the viral antigens, triggering a response dominated by cytotoxic T cells, a Th1 response (see Adaptive Immunity). Killed organisms and purified antigens, in contrast, commonly stimulate responses dominated by antibodies, a Th2 response. This antibody response may not generate optimal protection against some organisms. As a result, vaccines that contain killed organisms or purified antigens usually require the use of adjuvants to maximize their effectiveness. Adjuvants may, however, cause local inflammation, and multiple doses or high doses of antigen increase the risks of producing hypersensitivity reactions (see Adjuvants).