Babesia rodhaini: A study of the effects of immune serum

The effect of passively administered immune serum against B. rodhaini has been studied quantitatively. The curve relating the proportion of parasites surviving, to the dose of serum, is sigmoid with a steep middle region. Parasites are killed at a rate proportional to the dose of serum in the early stages of infection, but after about 24 h a residual population of parasites multiplies at a rate only slightly less than that of normal parasites in normal hosts. The slight reduction is attributed to a residual effect of immune serum, and to the induction of an active immune response inadequate to prevent continuing multiplication of parasites. The relationship between log 10 (parasite dose) and ST 50 for groups of mice is used to measure the dose of infective parasites.