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