Fundamentals of Immunology, Course 4: Dueling with the Dark Side, covers the constant battle between your immune system and threats to your health. The first two lectures discuss viral and cellular pathogens, focusing on specific tactics they use to deflect immune attack. Next, a cheerier lecture describes other immune-based therapies, beginning with the use of engineered antibodies to treat specific diseases. This lecture continues with vaccine development (viral-engineered, killed, RNA-component vaccines and more) and adjuvant function and ends with vaccination strategies, including herd immunity and resisting the anti-vaccination movement. The last two lectures cover immune deficiencies and how cancer evades the immune system, concluding with advances in using immune therapies (antibodies and CAR T cells) to cure cancer.