This session looks at the different types of white blood cells and their roles in defence. Students will stain blood smears for microscopic examination and see how flow cytometry can be used to identify and count different white blood cell populations. We shall also see how numbers of these cells can change in diseases such as HIV and leukaemia. Antibodies are large glycoproteins present in the blood and other body fluids that protect against infection, but which can also play a harmful role in some diseases. In this session we will see how antibodies can be detected in the laboratory and how this can be useful in diagnosis.