Which of the following stages is involved in the body’s response to an antigen?

Phagocytosis is a crucial stage in the body's immune response to an antigen. When the body encounters foreign substances, such as bacteria or viruses (referred to as antigens), immune cells known as phagocytes play an essential role in identifying and engulfing these pathogens. During phagocytosis, these immune cells surround the antigen and ingest it into a compartment within the cell, where it can be broken down and destroyed. This process not only helps eliminate the foreign invaders but also aids in presenting pieces of these antigens to other immune cells, thereby facilitating a more robust and specific immune response.

Understanding the importance of phagocytosis highlights its role in the overall defense mechanism, signifying that it is directly tied to how the body identifies and combats infections or foreign materials. The other processes mentioned, such as osmosis, diffusion, and filtration, are related to movement of substances across membranes or within fluids but do not specifically pertain to the body’s defense mechanisms against antigens.