A lecture about the disorders due to the Immune complex depositions. Antigen-antibody complexes activate complement that results in inflammatory damage to the nearby tissue. Serum sickness and Arhus reactions are discussed. Diseases discussed are: SLE, Rheumatoid Arthritis, Polyarteritis Nodosum, Post-streptococcal GN, Famer's lungs (hypersensitivity pneumonitis) etc.