American Airlines, Inc., operating as American Airlines (AA), is a U.S. airline and a subsidiary of AMR Corporation.t is headquartered in Fort Worth, Texas, adjacent to its largest hub at Dallas/Fort Worth International Airport. American Airlines operates an extensive international and domestic network, with scheduled flights throughout North America,â¦.
The major function of the immune system is to protect the host from environmental agents such as microbes or chemicals, thereby preserving the integrity of the body.
The immune system is a complex system of interacting cells whose primary purpose is to identify foreign (ânon-selfâ) substances referred to as antigens. The immune system provides protection from infectious disease by identify most of these microbes as foreign. Helper T-cells (CD4+) are middlemen in the immune response. 2  When they get activated, they proliferate and secrete cytokines that regulate or help effector lymphocyte function. 3  They are known as one of the targets of HIV infection, and the decrease of CD4+ T-cells results in AIDS.