Antibodies are remarkable for their diversity and specificity. They provide protection against many kinds of bacteria, especially those enclosed in a polysaccharide capsule, such as bacteria that cause pneumonia and meningitis. However, B cells and their antibodies have limitations. For example, B cells on their own can make antibodies against only some antigens; for other antigens, they require the assistance of other cells. Furthermore, they are sometimes ineffective against pathogens that take up residence inside a host cell, as do the bacteria that cause tuberculosis.

To handle these and other kinds of pathogens, there is another part of the adaptive immune system that depends on the second type of lymphocyte: the T cell. T cells do not secrete antibodies. Instead, they participate in cell-mediated immunity, so named because cells, not antibodies, recognize and act against pathogens. T cells target infected or diseased cells. Their importance is most dramatically demonstrated by individuals taking immunosuppressive drugs or infected by HIV, both of which suppress the activity of T cells. The result can be overwhelming infections and even tumors.