Final answer:
The statement is true; neutrophils, a type of leukocyte, circulate in the bloodstream and move to sites of infection or inflammation in response to chemokine signals through a process called chemotaxis.
Step-by-step explanation:
The statement 'Neutrophils are usually in the blood unless they sense chemokine signal and are directed towards connective tissue' is true. Neutrophils are a type of white blood cell, specifically a type of leukocyte, that are vital first responders to infection. They typically circulate within the bloodstream and are recruited to sites of infection or inflammation when they sense chemokine signals, a process known as chemotaxis. Once they receive these signals, they move from the blood into the connective tissue.
Neutrophils are not only phagocytic, meaning they can engulf and digest pathogens, but they also produce large quantities of cytokines which assist in pathogen destruction. This cellular behavior is part of the immune system's innate response, with neutrophils quickly arriving at sites of infection to begin the process of pathogen elimination.
The recruitment of neutrophils is a complex process involving chemical attractants released by pathogens and signals from nearby injured cells, as well as leukocytes already present at the site of damage or infection. These attractants guide the neutrophils out of the blood vessels and into the affected tissues through the endothelial barriers of the capillaries in a process referred to as positive chemotaxis.