Avian pathogenic Escherichia coli is a current problem in the poultry industry, causing mortality and economic losses. This paper evaluates the dynamics in immune response after the use of spray vaccination against E. coli and, thereby, seeks to understand how the vaccine can provide protection. During the early stages of response to vaccination the presence of antigen-presenting cells is predominant, but these diminish within the first 7 days after vaccination. The immune correlate of protection of vaccination using the E. coli vaccine Poulvac E. coli (aroA-deficient mutant strain) probably does not depend on the production of circulating antibodies (as assessed through the presence of B lymphocytes) and is linked to the presence of CD4+TCRVbeta1+. These cells act on mucosa tissue stimulating the production of immunoglobulin A. Vaccination stimulated a high state of immunocompetence, as assessed by measurement of several cellular subsets. This state of "immune alertness," however, may be associated with reduced weight gain. The high presence of naive and memory CD8 cells in the vaccinated group at 14 and 21 days postvaccination may indicate greater ability in the future to prevent tissue invasion by E. coli, based on the possibility that these cells will proliferate rapidly to a new stimulus. The simultaneous use of vaccine with the antibiotic ceftiofur sodium interferes with the immune response obtained through vaccination. In combination, the data obtained in this study indicate that the immune response produced by a spray vaccine against E. coli is mainly a cellular response, especially relevant to the sites in contact with the pathogen. It is suggested that there is a strong cell migration to the mucous membranes, where macrophages act first and then lymphocytes take part to protect the host. It is believed that recruited lymphocytes will act in the production of secreted IgA, which probably plays a greater role in the defense when compared with circulating immunoglobulins. The assessment of cellular dynamics by flow cytometry made it possible to elucidate the operation mechanism of the live E. coli vaccine.