What ELISPOT assays do and what they are good for

The ELISPOT technique is a revolutionary and exceptionally precise tool in immunology research.

ELISPOT assays are an increasingly common technique for detect tiny amounts of cytokines secreted by T lymphocytes in vitro. The ELISPOT technique involves stimulating the cells to secrete such that the cytokines are concentrated within culture wells. The cytokines then bind to complementary anti-cytokine antibodies coating the well bottoms. The cells are washed away, after which an enzyme-labeled anti-cytokine and a chromogenic substrate are used to create a graphical footprint of the cytokine secretions.

Loosely speaking, the number of footprints generated (i.e. the number of spots) indicates the number of secreting cells. I say "loosely speaking" because there is always the possibility of overlapping footprints. This problem is common when counting bacterial colonies as well.