Soon after that, Behring discovered antitoxins, and this seemed to favour the chemical or humoral theory of immunity. According to the latter, microbes and their poisons were rendered harmless by the chemical properties of the blood serum, properties similar to those of disinfecting substances.
In spite of his firm conviction of the solidity of the phagocyte theory, this discovery was a shock to Metchnikoff, for it was in apparent contradiction with the cellular theory of immunity. He hastened to undertake a series of researches; his overflowing eagerness infected his whole circle, every one taking the warmest interest in the progress of his experiments.
This was just as preparations were being made to take part in the London Congress, where the question of immunity was to be debated and had indeed been placed at the head of the programme. Many papers were being prepared, and a veritable tourney of opinions was to take place at this Congress.
Metchnikoff had already been to England once, in the spring of 1891, on the occasion of his reception as an Honorary Doctor by the University of Cambridge. This gave him the opportunity of making closer acquaintance with the English, who inspired him with great sympathy; years only increased this feeling. He appreciated the originality of their earnest and generalising spirit, their loyalty and energy; he was grateful to them for the attentive and favourable attitude with which his scientific work and himself had been received.
He was therefore delighted that this Congress, which was to be the scene of his final struggle against his contradictors, should take place in England and not in Germany, a country hostile to his ideas.
In view of the importance of the coming debate, a series of fresh experiments was made. This time Metchnikoff undertook them not only in person, but also in collaboration with M. Roux and with some students. The whole laboratory was in a state of effervescence.
The principal papers to be read at the Congress on the question of immunity were those of Messrs. Roux and Büchner, the first entirely in favour of the phagocyte theory and the second supporting the humoral theory.
Metchnikoff read an epitome of his researches and of his answers to attacks on his theory. Towards the end of the Congress the latter had visibly acquired the suffrage of numerous scientists. Roux wrote to me from London concerning my husband’s paper:
Metchnikoff is busy showing his preparations and, besides, he would not tell you how great is his triumph. He spoke with such passion that he carried everybody with him. I believe that, this evening, the phagocyte theory is the richer by many friends.