Suspension of Agglutinin Unites with its antigen II. cells causes its clumping together and settling out of suspension. Suspension of Opsonin Unites with its antigen II. cells and makes the cells (?) more easily taken up by phagocytes. Suspension of Amboceptor Unites with its antigen III. cells and also with complement which latter then dissolves the antigen. Precipitin Antiprecipitin Neutralizes precipitin. I. Agglutinin Antiagglutinin Neutralizes agglutinin. I. Opsonin Antiopsonin Neutralizes opsonin. I. Amboceptor Antiamboceptor Neutralizes amboceptor. I. (two kinds) Complement Anticomplement Neutralizes complement. I. SUMMARY OF IMMUNITY AS APPLIED TO PROTECTION FROM DISEASE. The discussion of “immunity problems” in the preceding chapters serves to show that protection from disease either as a condition natural to the animal or as an acquired state is dependent on certain properties of its body cells or fluids, or both. The actual factors so far as at present known may be summarized as follows: 1. _Antitoxins_ which neutralize true toxins; shown to exist for very few diseases. 2. _Cytolytic substances_ which destroy the invading organism: in reality two substances; amboceptor, which is specific, and complement, the real dissolving enzyme. 3. _Phagocytosis_ or the destruction of the invading organisms within the leukocytes. 4. _Opsonins_ which render the bacteria more readily taken up by the phagocytes. 5. _Enzymes_ other than complement possibly play a part in the destruction of some pathogenic organisms or their products. This remains to be more definitely established. 6. It is possible that in natural immunity there might be no receptors in the body cells to take up the organisms or their products, or the receptors might be present in certain cells but of a very low chemical affinity, so that combination does not occur. It is even highly probable that many substances formed by invading organisms which might injure specialized cells, such as those of glandular, nervous or muscle tissue, have a more rapid rate of reaction with, or a stronger affinity for, lower unspecialized cells, such as connective and lymphoid tissue, and unite with these so that their effects are not noticed. The importance of these different, factors varies in different diseases and need not be considered in this connection.