After the introduction of a disease, its aetiology is one of the most important topics to learn. The short video provides the aetiology of lymphocytosis. The video also provides basic knowledge of Acute lymphoblastic leukaemia. Histological differentiation of lymphocytosis and lymphoblastic leukaemia is also given. TIMELINE: 0:00 Introduction 0:46 What are lymphocytes? 1:14 Lymphatic system 1:27 What causes high lymphocyte count? 2:03 What is Acute lymphoblastic leukaemia? 2:35 Histological slides of lymphocytes 3:27 Histological slide of Acute lymphoblastic leukaemia