IMMUNOLOGY 841 



1934). The same types of cells are involved, and the extent to which 

 they are involved depends, as in other sites, upon the nature of the 

 foreign material or infectious agent. The heterophils are often mobilized 

 first, and the lymphoid-macrophage series shows the most pronounced 

 histological changes, with the macrophages seeming to bear the brunt of 

 the activity. Fibroblasts rarely come into contact with foreign material 

 in the blood and are rarely active. Endothelial cells, although they come 

 into contact with hematogenous material, show extremely little histologi- 

 cal change or phagocytic activity. 



The foregoing account of defense reactions, involving the disposal of 

 foreign material and tissue debris with eventual repair, is characteristic 

 when either antigenic or nonantigenic materials are introduced into a 

 normal animal, but certain quantitative differences are noted when anti- 

 genic materials, including parasites, are introduced into an immune 

 animal. These differences are chiefly due to antibodies and are specific. 

 When antigens are introduced into the immune body, they are generally 

 localized by agglutination if they are cells, or by precipitation if they are 

 in solution, and are made more readily phagocytable by opsonification. 

 Such localization and opsonification are particularly well seen in local 

 reactions in the loose connective tissue. They are often limited to organs 

 such as the spleen, liver, and bone marrow in general reactions, as is 

 well-illustrated in malaria. In trypanosomiasis, these phases have not 

 been completely studied, but ablastin (the reproduction-inhibiting anti- 

 body) at least does not involve either localization or phagocytosis. When 

 antigen and antibody meet in the tissues of an immune animal, not only 

 do localization and opsonification of the antigen occur, but there is 

 generally a much heightened inflammatory reaction (hypersensitivity, 

 sometimes evidenced by a local reaction — the so-called skin test — when 

 suitable amounts of antigen are injected intradermally). Provided this 

 heightened inflammation is not so intense as to overwhelm the body, 

 it represents a speeding up of the whole cellular response in the immune 

 animal. In addition to the specific action of antibodies after the acquisi- 

 tion of immunity, a residual increase in the number of cells of the 

 lymphoid-macrophage system sometimes is seen at strategic sites, which 

 results in a much more rapid mobilization of macrophages during im- 

 munity. This is well illustrated in the spleen in malaria. 



