1. Hemaggluttinin (HA) is an important surface structure protein of the influenza virus that is an essential gene for the spread of the virus throughout the respiratory tract. This enables the virus to attach itself to a cell in the respiratory system and penetrate it.
2. A substance, such as an antibody, that causes agglutination of red blood cells.
(2008, Geary W.Skick. Protecting your business in a pandemic : plans, tools, and advice for maintaining business continuity. ISBN: 978-0-313-34602-6)