The defence mechanism, often known as the immune system, protects against particular foreign elements.
Lymphocytes are the primary immune system cells.
They, like other blood corpuscles, develop from stem cells called hemocytoblasts, which are found in the fetal liver and adult bone marrow.
Some of them differentiate in the gut-associated bursal lymphoid tissues (tonsils, peyer patches, appendix) and are known as bursal or B lymphocytes, while others differentiate in the thymus gland and are known as thymus or T lymphocytes.
Mature lymphocytes enter the bodily fluids (blood and lymph) and circulate throughout the body.