immune system

Short definition from Wiktionary

Noun

1. 1
   The system that differentiates self from non-self and protects the body from foreign substances and pathogenic organisms by producing an immune response. It includes organs such as the thymus, the spleen and lymph nodes; tissue such as bone marrow, and lymphoid tissues such as the tonsils; cells such as lymphocytes including the B cells and T cells, and cell products such as antibodies.