The immune system is comprised of "innate" and "adaptive" immune systems and uses them to restore homeostasis in response to a disease.
The innate immune system is a non-specific response to the entry of pathogens in the body, that is, the response is generic and does not distinguish depending on the type of pathogen. The body either receives a signal from damaged/stressed cells or the pathogens are recognized by pattern recognition receptors. It has no memory, that is, if the same pathogens attack again, the response will be the same. In human beings, a specific type of innate immune system called the complement system is present. Here, a number of proteins that are present in our body (blood) complement the destruction of pathogens by antibodies.
The adaptive immune system, on the other hand, is pathogen-specific and has an immunological memory, that is, if the same pathogen attacks again, the previous response's history will be used to counter it effectively. Lymphocytes, a special group of cells, identify the pathogens and kill the damaged cells. Some of these cells are also converted to memory cells, to "remember" the pathogen attack and thus help the body in mounting a stronger defense in case the same pathogen is detected in the body during the lifetime of an organism. These memory cells are also transferred from a mother to a newborn baby, enabling a passive memory.
The most general response we see in our body is fever and control over the flow of fluids (excessive urination, etc.).