Leucocytes, also known as white blood cells, are an important component of the immune system. They play a crucial role in defending the body against infections and diseases. Out of the given options, option D is true, which states that leucocytes can squeeze out through the capillary walls. Let's explore this in more detail.

Leucocytes are produced in the bone marrow, which is a spongy tissue found inside certain bones. They are constantly being produced and released into the bloodstream to maintain a sufficient number of white blood cells in the body.

Leucocytes are able to leave the bloodstream and enter the surrounding tissues in response to infection or injury. This process is known as diapedesis or extravasation. During diapedesis, the white blood cells can squeeze out through the capillary walls and migrate towards the site of infection or injury.

This ability to squeeze out through the capillary walls is crucial for the white blood cells to effectively combat pathogens and initiate an immune response. Once they have entered the tissues, leucocytes can directly interact with pathogens, engulf them through a process called phagocytosis, or release chemicals to destroy them.

It is important to note that the sudden fall in the number of leucocytes is not necessarily an indication of blood cancer, as mentioned in option A. A decrease in the number of white blood cells can be caused by various factors, including infections, certain medications, autoimmune disorders, or bone marrow disorders. A proper medical diagnosis is necessary to determine the underlying cause of a decrease in leucocyte count.

In conclusion, leucocytes have the ability to squeeze out through the capillary walls, allowing them to migrate to sites of infection or injury in the body. This capability is crucial for their role in the immune response.