Some viral diseases are dangerous only when a person is infected for the first time; subsequent infections produce only mild symptoms or go entirely unnoticed. This pattern can be explained by the action of .

1. Memory cells that trigger a secondary immune response

2. Specific antibodies that remain concentrated in the bloodstream for life after the first infection event

3. Memory cells that trigger a primary immune response

4. T cells that trigger a primary immune response

1. Memory cells that trigger a secondary immune response

Explanation:

During the first infection when the body is invaded by a particular antigen, the immune system tries to identify the type of antigen. B lymphocytes make antibodies when the antigen has been identified in the event of a primary infection. The response of the immune system during the primary infection of that pathogen is usually slow. As soon as the primary infection is arrested by the antibodies produced in response, memory cells such as the memory B cells is formed, which enables the immune system to identify the same pathogen, and thereby triggers a faster secondary immune response in subsequent infections by the same pathogen.