Short answers, pulled from the story.
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to viruses, bacteria, parasites, cancer cells, and foreign objects such as wood splinters, distinguishing them from the organism's own healthy tissue.
The innate immune system gives an immediate but non-specific response and is found in nearly all forms of life. The adaptive immune system gives a tailored, antigen-specific response with immunological memory, and is found only in jawed vertebrates.
When B cells and T cells are activated and replicate, some offspring become long-lived memory cells that remember each pathogen encountered. If the same pathogen returns, these memory cells mount a faster and stronger response, which is the basis of vaccination.
Failures of immune defense fall into three categories: immunodeficiencies, autoimmunity, and hypersensitivities. Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus.
Through immune surveillance, the immune system identifies tumor cells by antigens not found on normal cells. Killer T cells lead the attack by recognizing tumor antigens presented on MHC class I molecules, while natural killer cells destroy tumor cells that display too few MHC class I molecules.
The earliest known reference to immunity was during the plague of Athens in 430 BC, when Thucydides noted that survivors could nurse the sick without falling ill again. In the 10th century the Persian physician al-Razi wrote the first recorded theory of acquired immunity, and Louis Pasteur later developed vaccination and the germ theory of disease.