An<unk>  is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could cause serious problems to the health of the host organism if not cleared from the body. There are two distinct aspects of the<unk> , the innate and the adaptive, which work together to protect against pathogens. The innate branch—the body's first reaction to an invader—is known to be a non-specific and quick<unk>  to any sort of pathogen. Components of the innate<unk>  include physical barriers like the skin and mucous membranes,<unk>  cells such as neutrophils, macrophages, and monocytes, and soluble factors including cytokines and complement. On the other hand, the adaptive branch is the body's<unk>  which is catered against specific antigens and thus, it takes longer to activate the components involved. The adaptive branch include cells such as dendritic cells, T cell, and B cells as well as antibodies—also known as<unk> globulins—which directly interact with antigen and are a very important component for a strong<unk>  against an invader. The first contact that an organism has with a particular antigen will result in the production of effector T and B cells which are activated cells that defend against the pathogen. The production of these effector cells as a result of the first-time exposure is called a primary<unk> . Memory T and memory B cells are also produced in the case that the same pathogen enters the organism again. If the organism does happen to become re-exposed to the same pathogen, a secondary<unk>  will kick in and the<unk>  system will be able to respond in both a fast and strong manner because of the memory cells from the first exposure. Vaccines introduce a weakened, killed, or fragmented microorganism in order to evoke a primary<unk> . This is so that in the case that an exposure to the real pathogen occurs, the body can rely on the secondary<unk>  to quickly defend against it.
Immune response
Humoral immunity
Muscular defense
Histocompatibility
Respiratory adaptation
Immune cycle
Drug metabolism
Respiratory burst