Leucocytes, commonly known as white blood cells, are a vital component of the immune system. They help the body fight infections and diseases by identifying and attacking harmful pathogens, such as bacteria and viruses. There are several types of leucocytes, including neutrophils, lymphocytes, and monocytes, each with specific functions in immune response. These cells are produced in the bone marrow and can be found in the bloodstream and lymphatic system. A healthy body typically maintains a balanced number of leucocytes, and abnormal levels can indicate various health issues, such as infections or blood disorders.