Question 12

Immunologists investigated the immune responses of healthy rats and rats with induced allergic airway inflammation (sensitised to the model antigen ovalbumin, OVA).

Rats in the control group were exposed to a nebulised aerosol of sterile phosphate-buffered saline (PBS) vehicle alone. Rats in the experimental group were exposed to OVA dissolved in PBS.

The immune response was recorded in two ways:

the cellular response by measuring the mean concentration of eosinophils in bronchoalveolar lavage (BAL) fluid (in $mL−110^4\text{ cells mL}^{-1}$ ) 48 hours after exposure.
the humoral response by measuring the mean concentration of OVA-specific IgE antibody (in $mL−1\mu\text{g mL}^{-1}$ ) in blood serum 14 days after exposure.

Table 1 shows the results of this investigation. The values in the brackets show $±2\pm 2$ standard deviations. A value of $±2\pm 2$ standard deviations from the mean includes over 95% of the data.

Table 1

Type of rats	Sex of rats	Mean eosinophil concentration in BAL fluid / $mL−110^4\text{ cells mL}^{-1}$	Mean concentration of OVA-specific IgE antibody / $mL−1\mu\text{g mL}^{-1}$
Healthy	Male	$(±1.6)8.4\ (\pm 1.6)$	$(±0.8)4.5\ (\pm 0.8)$
Healthy	Female	$(±1.2)9.2\ (\pm 1.2)$	$(±1.1)4.1\ (\pm 1.1)$
OVA-sensitised	Male	$(±3.1)14.5\ (\pm 3.1)$	$(±2.4)12.8\ (\pm 2.4)$
OVA-sensitised	Female	$(±4.2)22.6\ (\pm 4.2)$	$(±3.2)13.6\ (\pm 3.2)$

Suggest and explain one reason why the PBS vehicle on its own was administered to the control group of rats.

What can you conclude about the effects of OVA sensitisation on the cellular response and the humoral response in male and female rats? Use the data in Table 1 to justify your conclusions.