The immune system recognizes and eliminates foreign substances like microbes. It protects the body from old, damaged, and infected cells. The immune system has two branches - innate nonspecific immunity that acts quickly, and adaptive specific immunity that is highly targeted and develops memory. Antigens are substances that trigger immune reactions, while adjuvants enhance immune responses to antigens. Lymphocytes and accessory cells like macrophages and dendritic cells carry out immune functions in lymphoid organs like lymph nodes, spleen, and Peyer's patches.