Microbiology Practice Exam 2025 – Comprehensive All-in-One Guide for Exam Success!

The third line of defense in the immune response is primarily constituted by lymphocytes and antibodies. This aspect of the immune system is adaptive, meaning it develops in response to specific pathogens and involves a targeted attack against those invaders.

Lymphocytes, which include B cells and T cells, play a crucial role in this defense mechanism. B cells are responsible for producing antibodies that specifically bind to antigens on pathogens, neutralizing them or marking them for destruction by other immune cells. T cells, particularly cytotoxic T cells, directly attack infected or cancerous cells.

This adaptive response is characterized by its ability to remember previous infections, allowing for a quicker and more effective response upon subsequent exposures to the same pathogens. This memory aspect is what distinguishes the third line of defense from the innate immune responses represented in the other options, which are part of the first and second lines of defense. The first line includes physical barriers like skin and mucous membranes, while the second line involves various immune cells, such as macrophages and neutrophils, that respond to a wide range of pathogens but do not provide the specific enemy-identifying and attacking mechanism that antibodies do.