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The immune system comprises a complex network that includes various cells, tissues, organs, and organ systems that work together to defend the body against pathogens such as bacteria, viruses, and parasites. This system includes components such as white blood cells (like lymphocytes and macrophages), lymphatic tissues (such as lymph nodes and the spleen), and organs (such as the thymus and bone marrow). Each component has a specialized role in recognizing and responding to foreign invaders, making them essential for maintaining the body's health.

The other options refer to unrelated systems in the body. The network of nerves and hormones, for instance, relates to the nervous and endocrine systems, which control bodily functions but do not provide direct immune responses. Muscles and tendons are related to the musculoskeletal system, which is responsible for movement but does not involve immune defense. Lastly, joints and ligaments are also part of the musculoskeletal system, playing a role in structural support and movement, rather than in immune protection. Thus, the correct answer emphasizes the vital role of the specific and diverse components that constitute the immune system.