The basic defense against harmful agents is our immune system. Wikipedia says this about that system.
An immune system is a system of biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue.
Pathogens can rapidly evolve and adapt to avoid detection and destruction by the immune system. As a result, multiple defense mechanisms have also evolved to recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess a rudimentary immune system, in the form of enzymes that protect against bacteriophage infections. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and insects. These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system. Jawed vertebrates, including humans, have even more sophisticated defense mechanisms, including the ability to adapt over time to recognize specific pathogens more efficiently. Adaptive (or acquired) immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination.
Disorders of the immune system can result in autoimmune diseases, inflammatory diseases and cancer. Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections. In humans, immunodeficiency can either be the result of a genetic disease, such as severe combined immunodeficiency, or acquired conditions such as HIV/AIDS or the use of immunosuppressive medication. In contrast, autoimmunity results from an hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus. Immunology covers the study of all aspects of the immune system.
Scientists are actively researching our immune system. Here are some of their recent findings.
Early colonization of the gut by microbes in infants is critical for development of their intestinal tract and in immune development. A new study, published in BioMed Central’s open access journal Genome Biology, shows that differences in bacterial colonization of formula-fed and breast-fed babies leads to changes in the infant’s expression of genes involved in the immune system, and in defense against pathogens.