Lymphocytes are smaller than phagocytes and their nucleus fills most of the cell. They are produced before birth in the bone marrow.

T lymphocytes:

Migrate from bone marrow to the thymus (gland at base of breast bone) before they are mature. The thymus gland doubles in size between birth and puberty and then shrinks.

Have specific surface cell receptors (T cell receptors) which have similar structure to antibodies and are specific to one antigen.

T cells are activated when they meet an antigen in contact with host cell. This may be a macrophage with exposed antigen or a body cell that has been invaded by a pathogen and is displaying antigens on plasma membrane as a help signal.

Relevant T cells with matching receptors respond by dividing (mitosis).

There are three types of T cells:

Helper T cells:
Release cytokines (hormone like substance) when activated.
Cytokines stimulate right B cells to divide into plasma cells which make antibodies.
Cytokines stimulate macrophages to carry out phagocytosis more vigorously.

Killer T cells:
Search for cells invaded by pathogen and displaying antigens.
Attach to infected cells and secrete toxins e.g. hydrogen peroxide - kills body cell and pathogen.

Memory T cells:
These cause the response to be faster the second time round.