The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful pathogens, such as bacteria, viruses, fungi, and parasites. Its primary function is to identify and eliminate foreign invaders while distinguishing them from the body's own healthy cells. This ability to recognize self from non-self is crucial for maintaining overall health and preventing disease.

At the core of the immune system are two main types of immunity: innate immunity and adaptive immunity. Innate immunity provides the first line of defense and is present from birth. It includes physical barriers such as the skin and mucous membranes, as well as various immune cells, including macrophages and neutrophils, that respond quickly to infections. These components act in a non-specific manner, meaning they respond to a wide range of pathogens without prior exposure.

In contrast, adaptive immunity is a more specialized response that develops over time. It is characterized by the activation of lymphocytes, which include B cells and T cells. B cells are responsible for producing antibodies that specifically target pathogens, while T cells play a critical role in directly killing infected cells and regulating immune responses. This adaptive response is highly specific and improves upon repeated exposure to the same pathogen, leading to the formation of immunological memory. This memory allows the immune system to respond more rapidly and effectively during subsequent encounters with the same invader, which is the basis for vaccination.