An hypothesis to explain why cell‐mediated immunity alone can contain infections by certain intracellular parasites and how immune class regulation of the response against such parasites can be subverted

Summary Cells with a low density of parasite‐specific antigens on their surface are postulated to be susceptible to a cell‐mediated attack but not to effector mechanisms normally activated following the binding of specific antibody to the infected cell. It is further postulated that such infected cells normally induce a cell‐mediated response, and that cells infected with slow‐growing intracellular parasites have a low density of parasite‐specific antigens on their surface. Despite these general postulates, cell‐mediated immunity is not invariably induced following natural infection by certain slow‐growing parasites, such as those responsible for leprosy, tuberculosis, and the leishmaniases, and antibody can be induced that is exclusive of a strong, cell‐mediated response. It is proposed that certain events in such cases subvert the normal regulatory processes that control the class of immunity induced. In these cases, the parasite‐infected cells, bearing a low representation of parasite antigens, induce antibody even though they are not susceptible to antibody‐dependent effector mechanisms, and so they are not eliminated. In this case, chronic infection and uncontrolled growth of the parasiteoccurs, often with fatal consequences.