What is true about T-lymphocytes in mammals : -

T-lymphocytes are produced from stem cells in the bone marrow and mature in the thymus (not the thyroid). Bone marrow and thymus are the primary lymphoid organs; lymphoid tissues such as lymph nodes and spleen are secondary organs where lymphocytes interact with antigens.
They are responsible for cell-mediated immunity. The three main types are:
Cytotoxic (killer) T-cells: directly kill infected cells and cancer cells.
Helper T-cells: stimulate B-cells to make antibodies and activate other T-cells (these are the cells attacked by HIV).
Suppressor T-cells: suppress the immune response when it is no longer needed and prevent attack on the body's own cells.
Scavenging of damaged cells and debris is done by macrophages and neutrophils, not T-cells.
So the true statement is that there are three main types: cytotoxic, helper and suppressor T-cells.