How was the function of the immune system discovered? 

Smallpox (天花)：Caused by the variola [və'raiələ] (天花) virus. It enters the body through the lungs and is carried in the blood to the internal organs and the skin where it multiplies. 

More details on smallpox, check the following website:

https://www.medicinenet.com/smallpox/article.htm#smallpox_facts

The greatest killer in human history:

It was responsible for estimated 300–500 million deaths during the 20th century 

It causes blood poisoning, secondary (继发性) infections or internal bleeding in severe cases

The earliest disease found to induce life-long immunity.

Edward Jenner  爱德华·詹纳  (1749-1823)

Hedeveloped a safe vaccine for smallpox: cowpox (牛痘）

Edward Jenner tested vaccination against smallpox on various patients.

Etymology on Vaccination:

1800, used by British physician Edward Jenner (1749-1823) for the technique he devised of preventing smallpox by injecting people with the cowpox virus (variolae vaccinae), from vaccine (adj.) "pertaining to cows, from cows" (1798), from Latin vaccinus "from cows," from vacca "cow," a word of uncertain origin. "The use of the term for diseases other than smallpox is due to Pasteur" [OED].

Check the History of Vaccinations and Vaccines:

https://www.verywellfamily.com/information-and-history-about-vaccination-and-vaccines-2633706

The History Of Vaccines And Immunization: Familiar Patterns, New Challenges  By Alexandra Minna Stern and Howard Markel

https://www.healthaffairs.org/doi/full/10.1377/hlthaff.24.3.611

Read the short passage in “Lead-In”within 2 minutes and answer the following questions.

A wide variety of organisms and their associated molecules pose a constant threat to the human body. The human immune system—the defensive mechanisms that identify and neutralize these threats—is able to distinguish "nonself" organisms and molecules from "self", that which belongs within the body. Threats may enter the body from the outside (e.g., infectious organisms or toxic agents) or may arise from potentially harmful changes occurring within the body (e.g., the malignant transformation of a previously normal cell into a cancer cell). Fortunately, the immune system consists of three layers of defense. The first line of defense is provided by a set of mechanical, chemical, and biologic barriers that protect the body. If these barriers are breached , the second and third lines of protective systems are activated: first the innate immune system and then the adaptive immune system. 

Notes

nonself['nɔn'self] 

     n. any antigen-bearing foreign material that enters the body and normally stimulates an attack by the body's immune system (distinguished from self) （有机体的）异物；非我 

breach [bri:tʃ] 

     vt. to make a hole in a wall, fence, etc. so that sb/sth can go through it 在……上打开缺口 

innate immune system [i'neit,i'mju:n]天然免疫系统 

adaptive immune system  [ə'dæptiv]  适应性免疫系统