Questions: Once a helper T-lymphocyte recognizes an antigen, it begins the immune response by secreting chemical signals called Multiple Choice memory T-lymphocytes. memory B-lymphocytes. immunoglobulins. antibodies. cytokines.

The answer is the last one (E): cytokines.

Explanation for each option:

1. Memory T-lymphocytes: These are a type of T cell that remains in the body after an infection has been cleared to provide a faster response if the same antigen is encountered again. They do not secrete chemical signals to initiate the immune response.

2. Memory B-lymphocytes: These are a type of B cell that also remains in the body after an infection to provide a quicker response upon re-exposure to the same antigen. They are not involved in secreting chemical signals to start the immune response.

3. Immunoglobulins: These are antibodies produced by B cells. While they play a crucial role in the immune response by binding to antigens, they are not the chemical signals secreted by helper T-lymphocytes.

4. Antibodies: These are proteins produced by B cells that specifically bind to antigens to neutralize them or mark them for destruction. They are not secreted by helper T-lymphocytes.

5. Cytokines: These are the chemical signals secreted by helper T-lymphocytes. They play a critical role in regulating the immune response by signaling other immune cells to act.

Summary: Helper T-lymphocytes secrete cytokines to initiate and regulate the immune response.