Immunology is the study of the immune system and its functions of protecting the body from infectious organisms like viruses, bacteria, fungi, and parasites. The immune system is a complex system consisting of organs, molecules, cells, and pathways that work together to fight pathogens, provide protection, and distinguish self from non-self. Antigens are substances that can generate an immune response by binding to immune cells and antibodies. Immunogens are antigens that are capable of eliciting an immune response, while haptens are small molecules that need a carrier like a protein to elicit an antibody response.