Cell-Mediated Response

Cell-mediated response is the specific immune response that utilizes T cells to neutralize cells that have been infected with viruses and certain bacteria. “Cell-mediated” refers to the fact that the response is carried out by cytotoxic T cells. T cells are so named because they mature in the thymus. There are three types of T cells: cytotoxic, helper, and suppressor T cells. Cytotoxic T cells destroy virus-infected cells in the cell-mediated immune response, and helper T cells play a part in activating both the antibody and the cell-mediated immune responses. Suppressor T cells deactivate T cells and B cells when needed, and thus prevent the immune response from becoming too intense.

The innate immune system contains cells that detect potentially harmful antigens, and then inform the adaptive immune response about the presence of these antigens. An antigen-presenting cell (APC) is an immune cell that detects, engulfs, and informs the adaptive immune response about an infection. When a pathogen is detected, these APCs will phagocytose the pathogen and digest it to form many different fragments of the antigen. Antigen fragments will then be transported to the surface of the APC, where they will serve as an indicator to other immune cells. Macrophages and dendritic cells are two examples of leukocytes that function as APCs. Helper T cells are one of the main lymphocytes that respond to APCs.

When APCs phagocytize pathogens and present antigens on the surface, the cytotoxic T cells become activated. These resulting cytotoxic t cells then identify other cells with the same antigens, thus identifying pathogens and infected host cells. Cytotoxic T cells attempt to identify and destroy infected cells before the pathogen can replicate and escape, thereby halting the progression of infections. Cytokines secreted by helper T cells stimulate cytotoxic T cells to enhance their ability to identify and destroy infected cells and tumors.

B cells are part of the humoral immune response. B cells are so named because the are produced by bone marrow. Humoral immunity is immunity from serum antibodies produced by plasma cells. More specifically, someone who has never been exposed to a specific disease can gain humoral immunity through administration of antibodies from someone who has been exposed, and survived the same disease. “Humoral” refers to the bodily fluids where these free-floating serum antibodies bind to antigens and assist with elimination. Antibodies in the body can bind to antigens directly, whereas T cell receptors can only recognize antigens that are bound to certain receptor molecules, called Major Histocompatibility Complex (MHC). proteins (this is why this mechanism is called the cell-mediated response).

When a naive B cell encounters an antigen that fits or matches its membrane-bound antibody, it quickly divides in order to become either a memory B cell or an effector B cell, which is also called a plasma cell. Antibodies can bind to antigens directly. Plasma cells can secrete antibodies, which identify free pathogens that are circulating throughout the body. When the naive B cell divides and differentiates, both plasma cells and memory B cells are made.

Immunological Memory

The adaptive immune system possesses a memory component that allows for an efficient and dramatic response upon reinvasion of the same pathogen. Memory is handled by the adaptive immune system with little reliance on cues from the innate response. During the adaptive immune response to a pathogen that has not been encountered before, called a primary response, plasma cells secreting antibodies and differentiated T cells increase, then plateau over time. As B and T cells mature into effector cells, a subset of the naïve populations differentiates into B and T memory cells with the same antigen specificities.

A memory cell is an antigen-specific B or T lymphocyte that does not differentiate into effector cells during the primary immune response, but that can immediately become effector cells upon reexposure to the same pathogen. During the primary immune response, memory cells do not respond to antigens and do not contribute to host defenses. As the infection is cleared and pathogenic stimuli subside, the effectors are no longer needed, and they undergo apoptosis. In contrast, the memory cells persist in the circulation.

A good example of immunological memory is shown in vaccinations. Vaccination is based on the knowledge that exposure to noninfectious antigens, derived from known pathogens, generates a mild primary immune response. The immune response to vaccination may not be perceived by the host as illness but still confers immune memory. When exposed to the corresponding pathogen to which an individual was vaccinated, the reaction is similar to a secondary exposure. A vaccination against a virus can be made using either active, but weakened or attenuated virus, or using specific parts of the virus that are not active. Both attenuated whole virus and virus particles cannot actually cause an active infection. Instead, they mimic the presence of an active virus in order to cause an immune response, even though there are no real threats present. By getting a vaccination, you are exposing your body to the antigen required to produce antibodies specific to that virus, and acquire a memory of the virus, without experiencing illness.

Because each reinfection generates more memory cells and increased resistance to the pathogen, and because some memory cells die, certain vaccine courses involve one or more booster vaccinations to mimic repeat exposures. For instance, tetanus boosters are necessary every ten years because the memory cells only live that long.

In the competition between immune protection and pathogen evasion, pathogens have the advantage of more rapid evolution because of their shorter generation time and other characteristics. For instance, Streptococcus pneumoniae (bacterium that cause pneumonia and meningitis) surrounds itself with a capsule that inhibits phagocytes from engulfing it and displaying antigens to the adaptive immune system.