In a drug trial, one patient receives an injection containing inactivated antigen A and another patient receives a saline injection. The immune system can mount a response against inactive antigen A. If each patient was exposed to antigen A twice after receiving the initial injection, which of the following graphs shows the expected changes in the levels of serum antibodies against antigen A across time?

Question

In a drug trial, one patient receives an injection containing inactivated antigen A and another patient receives a saline injection.  The immune system can mount a response against inactive antigen A.  If each patient was exposed to antigen A twice after receiving the initial injection, which of the following graphs shows the expected changes in the levels of serum antibodies against antigen A across time?

Quizplus · Accepted Answer

11ecba2d_1034_99f0_a3bf_cbe1d857fd8d_MD0008_00 Cells of the adaptive immune system recognize specific foreign antigens and mount specialized immune responses against them. Specifically, B lymphocytes secrete antibodies that circulate throughout the blood and lymph and bind foreign antigens, marking them for destruction by other immune cells. Upon binding a specific foreign antigen, a B lymphocyte becomes activated and engulfs (endocytoses) the foreign antigen, breaking it down into smaller fragments. Within the B lymphocyte, specialized antigen-binding peptides known as major histocompatibility class II (MHC II) proteins transport the antigen fragments to the cell surface, allowing the B lymphocyte to display the antigen fragments outside the cell. Helper T cells bind foreign antigens displayed by B lymphocytes and release signaling molecules that stimulate B lymphocytes to divide and differentiate into both short-lived, antibody-secreting plasma cells and long-lived memory B cells. Memory B cells remain in lymphoid tissues for a long time and can recognize and respond to the foreign antigen more rapidly in the event of a future infection. In this scenario, one patient initially receives a saline injection and the other patient receives an injection containing inactivated antigen A, which can be detected by the immune system. Injection of inactivated antigen A into a patient will allow B lymphocytes to bind the antigen, leading to the proliferation of both plasma cells and memory B cells. If future exposure occurs, these memory B cells will likely respond more rapidly to the antigen, increasing antibody production. In contrast, the saline-treated patient is not exposed to inactive antigen A, and therefore never produces memory B cells following the saline injection. However, upon the first post-injection exposure to antigen A, this patient will also produce antibodies against antigen A, but this response will be slower than in the patient initially injected with inactivated antigen A (Choices C and D). This first-time exposure to antigen A will lead to the proliferation of memory B cells that can more rapidly produce antibodies following a second exposure (Choice A). Educational objective: Upon binding foreign antigens, activated B lymphocytes divide and differentiate into antibody-secreting plasma cells and long-lived memory B cells that can more rapidly respond to foreign antigens in the event of future infections.

In a Drug Trial, One Patient Receives an Injection Containing