Step 1: Understanding the types of cells.
T cells and B cells are types of white blood cells involved in the immune response. Both are classified as lymphocytes.
Step 2: Analyzing the options.
(A) lymphocytes: Correct — T cells and B cells are both types of lymphocytes, a category of white blood cells.
(B) erythrocytes: Incorrect — Erythrocytes are red blood cells, not involved in immunity.
(C) epithelial cells: Incorrect — Epithelial cells form tissues lining organs but do not play a direct role in immunity.
(D) squamous cells: Incorrect — Squamous cells are a type of epithelial cell.
Step 3: Conclusion.
The correct answer is (A) lymphocytes.