Affinity (from lat. affinitas - relationship, relationship - the relationship, mutual relationship between two objects. In biology under the affinity is usually understood as the ability of cells and tissues to selectively capture of environmental certain substances (for example, A. fabric grey matter of the brain to tetanus toxin A. thyroid tissue and some malignant education to iodine). A. the different tissues of the same substance as A. the same histological formations of different animals to a particular agent, usually different and is determined by their structure, function and physical-chemical processes proceeding in them and in the environment. Affinity tissue to this or that substance can vary significantly as a result of direct impact on her or modify the reactivity of the organism. In the basis of primary pharmacological reactions of tissue on medicinal substances and reactions of various pathogens to antimicrobial drugs is A. tissues and pathogens to the appropriate agents. In the basis of resistance of bacteria and protozoa to certain specific substances in some cases is the reduction of their affinity to these substances. It should be noted that A. characterizes only the ability of the tissue or organism to capture a certain substance from the environment, but does not reflect their sensitivity to it.
Under A. understand the relationship between individuals or smaller groups depending on the similarity of their structure and function and points to the commonality of their origins, the similarities between individual organisms defined by their relationship (so-called genetic affinity). The term " morbid affinity" means a property of certain pathological manifestations to meet with several diseases simultaneously or in sequence. In addition, the distinction A. chemical (force behind the inclusion of an atom in the molecule), residual, or residual (power linking molecules), selective, or election (property elements to join in combination with some items that are not engaging in communication with others), and electric power (force with which the atoms retain their ion charges).