How does the human immune system recognize foreign invaders?

The immune system recognizes foreign invaders through a system of molecular identification. Every cell and pathogen displays proteins on its surface called antigens. Immune cells, particularly lymphocytes, carry receptors that can bind to specific antigens. B cells produce antibodies that match and bind to foreign antigens, while T cells recognize fragments of antigens presented by infected cells or immune cells called antigen-presenting cells. The key distinction is between "self" and "non-self." During development, the immune system learns to tolerate the body's own proteins. When receptors encounter unfamiliar molecular patterns, such as bacterial cell wall components or viral proteins, they trigger an immune response, leading to the destruction or neutralization of the invader.