A leucocyte, commonly known as a white blood cell, is a crucial component of the immune system. These cells help the body fight infections and diseases by identifying and attacking pathogens like bacteria and viruses. There are several types of leucocytes, including neutrophils, lymphocytes, and monocytes, each with specific roles in immune response. Leucocytes are produced in the bone marrow and circulate throughout the body in the bloodstream and lymphatic system. Their count can vary based on health conditions; for instance, an increased number may indicate an infection, while a decreased count can suggest a weakened immune system or other medical issues.