Question 4 To detect a humoral immune response to E. Coli bacteria, you would measure. __ a. cytotoxicity of virus-infected cells in the lung. b. dividing T cells in the draining lymph nodes. c. plasma cytokine levels. d. serum antibody titer.

# Explanation 1<br />The humoral immune response is a part of the immune response that involves the production of antibodies by B cells. These antibodies are specific to the antigen that triggered the immune response. <br /><br />## Step 2<br />In the context of an immune response to E. Coli bacteria, the humoral immune response would involve the production of antibodies specific to the E. Coli bacteria. <br /><br />## Step 3<br />To detect this humoral immune response, we would measure the serum antibody titer. The serum antibody titer is a measure of the concentration of antibodies in the blood. <br /><br />## Step 4<br />The other options, such as cytotoxicity of virus-infected cells in the lung, dividing T cells in the draining and plasma cytokine levels, are more related to cell-mediated immunity or the overall inflammatory response, rather than the humoral immune response.<br /><br /># Answer: <br />'d'