The blood plasma leaks into tissues during circulation. This is collected by the lymphatic system and is called lymph.
Lymph is very similar to blood plasma. It contains lymphocytes and other white blood cells. Lymphocytes are concentrated in the lymph nodes.
The lymph transports antigen presenting cells like dendritic cells to the lymph nodes.
Cells in the lymphatic system react to antigens presented or found. When an antigen is recognised an immunological cascade begins.
It involves the activation of immune system and production of antibodies and cytokines. It also leads to recruitment of immunological cells like macrophages.