Toothbrush brushing teeth.

Illustration showing screaming man with a lightning bolt striking through his face.

Medical illustration showing red blood cell in capillary with neurons in the background.

Medical illustration showing the brain with neurons around it.

Medical illustration of oxygen free radicals. After blood flow is restored to injured cells and tissues, the damaged cells produce oxygen free radicals, molecules which begin a process called lipid peroxidation, destroying cells around an injury. Here, the cell membrane lipid bilayer (blue) is being damaged by oxygen free radicals (red and white clusters).

Prevention of hypoxic hypoxemia.

Gastric tumor with involvement of perigastric and distal lymph nodes.

Peptic ulcer disease: Helicobacter pylori at the site of gastritis, polymorphonuclear leukocyte infiltration, and peptic ulceration.

Cell death: necrosis (left) vs. apoptosis. In necrosis, a response to cell damage or injury, the cell swells and its membrane ruptures, resulting in inflammation. In apoptosis, or programmed cell death, cells shrink and condense into multiple small membrane-bound bodies. In both processes, macrophages consume the remains.

Illustration of gene recombination within a B cell. This natural process involves the rearrangement of the genes encoding immunoglobulin proteins during B cell development, giving rise to antibody diversity. A similar process occurs in T cells, and accounts for T cell receptor diversity.

Medical illustration of the process of leukocyte diapedesis, which is the passage of white blood cells through intact capillary walls into the tissues. Here this emigration is in response to the presence of bacteria that the leukocytes will defend the body against.

Medical illustration of the process of making the hepatitis B vaccine. DNA from the hepatitis B virus is first isolated. It is then introduced into E. coli bacteria, which replicate large amounts of the virus protein. This protein is then used to make the vaccine, which when injected, activates immune response.

Medical illustration of white blood cells arming themselves against bacteria. Defense mechanisms involve antigen presentation by macrophages to helper T cell receptors and cytokine release by T cells. Also shown are the transformation of B cells into plasma cells that produce antibodies, and complement activation.

Medical illustration of white blood cells arming themselves against parasites. Macrophages phagocytize the parasites, then release the cytokine interleukin-1, which helps activate helper T cells. Macrophages present parasite antigens to the T cells, which release the cytokine interferon-gamma, enhancing phagocytosis.

Medical illustration of a virus entering a cell, and a killer T cell attacking this infected cell. The infected cell expresses viral proteins on its surface, enabling killer T cells to recognize it. The killer T cells then release perforin, which punctures the cell membrane, and lymphotoxin, which helps induce apoptosis.

Medical illustration of the mechanisms of an allergic reaction. Allergens, having entered the body, bind to both dendritic cells and B cells. T cells release cytokines that transform B cells into plasma cells, producing antibodies. Mast cells and inflammatory cells release the agents that cause the allergic reaction.

Medical illustration of various different types of cells of the immune system. Featured here are plasma cells, neutrophils, T lymphocytes, follicular dendritic cells, dendritic cells, natural killer cells, basophils, macrophages, mast cells, and eosinophils.

Illustration of a theory for making a super vaccine. Genes from various different viruses are recombined to create an artificial virus, which is then used to make the super vaccine. One dose of this genetically modified super vacccine should confer immunity against a host of illnesses.

Multiple sclerosis: illustration of axon demyelination in the central nervous system caused by immune system attack. Demyelination slows or blocks the normal transmission of nerve impulses, leading to the sensory, motor, and autonomic symptoms that characterize this disease.

Illustration of the immune response in the rejection of a transplanted organ (here a kidney). To lessen chances of rejection, donors and recipients who share as many major histocompatibility complex (MHC) genes as possible are matched up, and the recipient is put on immunosuppressive drugs.