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

a. memory cells that trigger a secondary immune response
b. specific antibodies that remain concentrated in the bloodstream for life after the first infection event
c. memory cells that trigger a primary immune response
d. t cells that trigger a primary immune response

a. memory cells that trigger a secondary immune response

Explanation:

Memory B and T cells are the long-lived immune cells that trigger a rapid and stronger immune response upon the subsequent encounter to a particular antigen.

Clonal selection in lymphocyte produces effector cell s and memory cells. If the same antigen or pathogen enters the body again, the already present memory cells stimulate a quicker and stronger immune response than the one generated during the first encounter. There are rapid proliferation and differentiation of memory cells into the effector and even more memory cells resulting in a vigorous immune response.

For example, during the first encounter with a pathogen, the proliferation of B cells produce antibody-producing plasma cells and memory B cells.