The lymphatic system of organs and vessels is of decisive importance for the immune defence. Important defence cells, primarily the lymphocytes, operate in the widely branching transport network which goes through the entire body. The white blood cells help to render undesired intruders such as viruses, bacteria and fungi harmless.

Primary Lymphatic Organs
The lymphocytes develop in the primary lymphatic organs, the bone marrow and the thymus gland. All of the lymphocytes develop in the bone marrow, B cells mature in the bone marrow, T cells in the thymus gland.

Secondary Lymphatic Organs
Tissue in which the defence cells become active is known as secondary lymphatic organs. They include lymph nodes, tonsils, the spleen and Peyer's plaques in the small intestine.