What type of immune response primarily characterizes transplant rejection?

• A. Humoral immunity
• B. Cell-mediated immunity
• C. Innate immunity
• D. Macrophage activation

Answer: (B)

The primary characteristic of transplant rejection is largely attributed to cell-mediated immunity. This type of immune response involves T lymphocytes, particularly cytotoxic T cells, which recognize and attack foreign antigen-presenting cells from the transplanted tissue. The immune system distinguishes the transplanted organ as non-self due to the presence of mismatched human leukocyte antigens (HLAs) between the donor and recipient. As a result, the T cells become activated and initiate a series of immune responses aimed at destroying the transplanted cells.

While humoral immunity (involving B cells and antibodies) can also play a role in rejection, particularly in acute rejection scenarios, cell-mediated immunity is predominantly responsible for both acute cellular rejection and chronic rejection processes. Innate immunity is more generalized and involves the first line of defense mechanisms, such as macrophages and neutrophils, but does not specifically target the precise nature of the transplanted tissue rejections. Macrophage activation, while potentially contributing to the immune response, is also part of the broader innate immune system and not the primary mechanism in transplant rejection.

Therefore, understanding that cell-mediated immunity is the principal pathway utilized in transplant rejection elucidates why this is the most accurate answer to the question.