Something that is foreign to your body and that induces an immune response or is the target of an immune response is called an antigen.
Almost any foreign molecule (non-self), and sometimes molecules that are part of the body (self), can act as antigens to our immune systems. Some examples of non-self antigens include viruses, bacteria, parasites, snake venom, food components (allergens) like egg white, things that may be injected into the body such as blood of a non-matching type, any many, many other things.