Most cases of scarlet fever occur in children. Adults have usually developed immunity to a toxin that the Streptococcus bacteria produce during infection. Explain how an adult develops immunity.

The adult was exposed to the toxin when they were a child, this developed an immune response. This immune response stimulated B lymphocytes to proliferate, these then made memory B lymphocytes. The memory B lymphocytes stay in the body for a very long time and induce a quicker response when the toxin is met later in life - they have developed immunity.