An immune response is generally divided into innate and adaptive immunity. Innate immunity occurs immediately, when circulating innate cells recognize a problem. Adaptive immunity occurs later, as it relies on the coordination and expansion of specific adaptive immune cells. Immune memory follows the adaptive response, when mature adaptive cells, highly specific to the original pathogen, are retained for later use.
Innate immune cells express genetically encoded receptors, called Toll-like receptors (TLRs), which recognize general danger- or pathogen-associated patterns. Collectively, these receptors can broadly recognize viruses, bacteria, fungi, and even non-infectious problems. However, they cannot distinguish between specific strains of bacteria or viruses.
There are numerous types of innate immune cells with specialized functions. They include neutrophils, eosinophils, basophils, mast cells, monocytes, dendritic cells, and macrophages (See Immune Cells for detailed descriptions). Their main feature is the ability to respond quickly and broadly when a problem arises, typically leading to inflammation. Innate immune cells also are important for activating adaptive immunity. Innate cells are critical for host defense, and disorders in innate cell function may cause chronic susceptibility to infection.
Adaptive immune cells are more specialized, with each adaptive B or T cell bearing unique receptors, B-cell receptors (BCRs) and T-cell receptors (TCRs), that recognize specific signals rather than general patterns. Each receptor recognizes an antigen, which is simply any molecule that may bind to a BCR or TCR. Antigens are derived from a variety of sources including pathogens, host cells, and allergens. Antigens are typically processed by innate immune cells and presented to adaptive cells in the lymph nodes.
The genes for BCRs and TCRs are randomly rearranged at specific cell maturation stages, resulting in unique receptors that may potentially recognize anything. Random generation of receptors allows the immune system to respond to new or unforeseen problems. This concept is especially important because environments may frequently change, for instance when seasons change or a person relocates, and pathogens are constantly evolving to survive. Because BCRs and TCRs are so specific, adaptive cells may only recognize one strain of a particular pathogen, unlike innate cells, which recognize broad classes of pathogens. In fact, a group of adaptive cells that recognize the same strain will likely recognize different areas of that pathogen.
If a B or T cell has a receptor that recognizes an antigen from a pathogen and also receives cues from innate cells that something is wrong, the B or T cell will activate, divide, and disperse to address the problem. B cells make antibodies, which neutralize pathogens, rendering them harmless. T cells carry out multiple functions, including killing infected cells and activating or recruiting other immune cells. The adaptive response has a system of checks and balances to prevent unnecessary activation that could cause damage to the host. If a B or T cell is autoreactive, meaning its receptor recognizes antigens from the body's own cells, the cell will be deleted. Also, if a B or T cell does not receive signals from innate cells, it will not be optimally activated.
Immune memory is a feature of the adaptive immune response. After B or T cells are activated, they expand rapidly. As the problem resolves, cells stop dividing and are retained in the body as memory cells. The next time this same pathogen enters the body, a memory cell is already poised to react and can clear away the pathogen before it establishes itself.
Vaccination, or immunization, is a way to train your immune system against a specific pathogen. Vaccination achieves immune memory without an actual infection, so the body is prepared when the virus or bacterium enters. Saving time is important to prevent a pathogen from establishing itself and infecting more cells in the body.
An effective vaccine will optimally activate both the innate and adaptive response. An immunogen is used to activate the adaptive immune response so that specific memory cells are generated. Because BCRs and TCRs are unique, some memory cells are simply better at eliminating the pathogen. The goal of vaccine design is to select immunogens that will generate the most effective and efficient memory response against a particular pathogen. Adjuvants, which are important for activating innate immunity, can be added to vaccines to optimize the immune response. Innate immunity recognizes broad patterns, and without innate responses, adaptive immunity cannot be optimally achieved.
Read more about Vaccines.