The figure shows the concentration of antibodies for an antigen produced by Clostridium tetani, a bacterium that causes tetanus. What could trigger the increase in antibody production marked by the arrow? 
Definitions:
Zero-Investment Portfolio
A portfolio strategy that involves creating a position without the need for initial capital by taking both long and short positions in stocks or other securities.
Net Investment
The total amount spent by a company on capital assets, such as plants and machinery, minus depreciation.
Nonsystematic Standard Deviation
A measure of the dispersion of returns on an investment due to factors specific to an individual asset or company, not the market.
Equally-Weighted Portfolio
An investment portfolio in which each asset is invested in with an equal amount of capital, providing diversification and reducing risk.
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