Human Immune System

The human immune system is a complex network of cells, tissues, and organs that defends the body against pathogens (bacteria, viruses, fungi, parasites) and internal threats (cancer cells). It has two components: the innate immune system (rapid, non-specific defense — inflammation, fever, natural killer cells) and the adaptive immune system (slower, specific — T cells and B cells that create tailored antibodies and immunological memory). Immunological memory is the basis of vaccines — a first exposure to a pathogen (or vaccine) trains the immune system to respond rapidly to future exposures. T cells destroy infected cells; B cells produce antibodies (Y-shaped proteins) that neutralize specific pathogens. Autoimmune diseases occur when the immune system attacks the body's own cells (rheumatoid arthritis, lupus, type 1 diabetes, multiple sclerosis). Immunotherapy (harnessing the immune system to fight cancer) is the most promising cancer treatment advance in decades.