Complement mediated release of histamine from human leukocytes

Activation of either the alternative or classical pathway of complement generated a factor which induced release of histamine from both non allergic and allergic human basophils. This factor probably is derived from the complement system since its formation was associated with loss of C3 activity in human serum, chemotactic factor (probably also a complement product) was generated simultaneously, heat inactivation blocked its formation, anti C3 and anti C5 blocked formation of the factor, and finally anti C5 inhibited the activity of the factor once it had been formed. It appears that both complement mediated and allergen mediated release of histamine from basophils are secretory, noncytolytic pathways since both were maximal at 37°C, required the presence of divalent cations, and were inhibited by theophylline. One consistent difference between these two mechanisms was noted: complement initiated release of histamine occurred more quickly.