Warning: Wikipedia contains spoilers.
In the novel, renegade biotechnologist Vergil Ulam creates simple biological computers based on his own lymphocytes. Faced with orders from his nervous employer to destroy these, he injects them into his own body. There the lymphocytes multiply and evolve rapidly, altering their own genetic material and quickly becoming self-aware. The nanoscale civilisation they construct soon begins to transform Ulam, then others, until eventually assimilating most of the biosphere of North America. This civilisation, which incorporates both the evolved human lymphocytes and recently-assimilated conventional humans eventually quits the normal plane of existence. This is one of the more extreme cases of the technological singularity found in science-fiction literature.
The book's structure (its sections are titled telophase, anaphase, etc.) mirrors the major phases of mitosis, a metaphor for the splitting off of the new lympho-creature civilisation from humanity. The novel has been critized for overestimating the speed and effectiveness of the lymphocites' growth and evolution, and for the apparent ease with which they become intelligent and with which they construct their immense civilisation. However, the realistic treatment of biotechnology and the depiction of existence in a subjective realm where one's consciousness can be cloned and modified may make this a prescient novel. It anticipates themes which were tackled by the postcyberpunk generation of writers. In particular, it is reminiscent of Permutation City, the Greg Egan novel which deals with computer-based consciousness.