The flu vaccine activates adaptive immunity. how does this occur? the immune system identifies antigens on the weakened flu virus. the actual flu virus enters the body, and lymphocytes recognize the antigens. antibodies are produced, which bind to the weakened flu virus and signal immune cells to destroy it. antibodies are quickly produced and allow the body to fight off the infection. a vaccine introduces a weakened flu virus into the body. ↓ ↓ ↓ ↓

Correct

A vaccine is a biological preparation that stimulates the antibodies production by activating the T lymphocytes and B lymphocytes and helps to provide immunity against one or more than one disease.

The vaccine is prepared by taking a small amount of dead or whole weakened germs such as bacteria or virus that cause the disease.

When these attenuated or disarmed pathogen are introduced in the body they evoke the immune response inside the body, which start preparing  antibodies against the particular antigen in advance in order to save the body from further  infection by the same pathogen .

A vaccine introduces a weakened flu virus into the body

                                                 ↓

The immune system identifies antigens on the weakened flu virus

                                                  ↓

Antibodies are produced, which bind to the weakened flu virus and signal immune cells to destroy it

                                                   ↓

The actual flu virus enters the body, and lymphocytes recognize the antigens

                                                   ↓

Antibodies are quickly produced and allow the body to fight off the infection

The correct order would be:

A vaccine introduces a weakened flu virus into the body.

                                                       ↓

The immune system identifies antigens on the weakened flu virus.

                                                       ↓

Antibodies are produced, which bind to the weakened flu virus and signal immune cells to destroy it.

                                                       ↓

The actual flu virus enters the body, and lymphocytes recognize the antigens.

                                                       ↓

Antibodies are quickly produced and allow the body to fight off the infection.

Explanation:

PLATO

substitution is the answer in this case