Answer the following question given this paragraph: 

Lymphocytes are the key cells in the immune response. They are leukocytes that become activated by a particular antigen. There are two major type of lymphocytes: B cells and T cells.


Q: What are the key cells in the immune response?

 Choices:

- keratinocytes

- erythrocytes

- leukocytes

- lymphocytes

A:
lymphocytes
