The immune system is a host defense system comprising many biological structures and processes within an organism that protects against disease. To function properly, a system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms. In many species, the system can be classified into subsystems, such as the innate immune system versus the adaptive immune system. In humans, the barrier, blood–cerebrospinal fluid barrier, and similar fluid–brain barriers separate the peripheral immune system from the neuroimmune system. 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 invertebrates. 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 immunity creates immunological memory after a response to a specific 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 system is less active than normal. In humans, immunodeficiency can either be the result of a disease such as severe combined immunodeficiency, acquired conditions such as HIV/AIDS. In contrast, autoimmunity results from an immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimotos thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, immunology covers the study of all aspects of the immune system. Immunology is a science that examines the structure and function of the immune system and it originates from medicine and early studies on the causes of immunity to disease. The earliest known reference to immunity was during the plague of Athens in 430 BC, thucydides noted that people who had recovered from a previous bout of the disease could nurse the sick without contracting the illness a second time. In the 18th century, Pierre-Louis Moreau de Maupertuis made experiments with scorpion venom and observed that certain dogs and this and other observations of acquired immunity were later exploited by Louis Pasteur in his development of vaccination and his proposed germ theory of disease. Pasteurs theory was in opposition to contemporary theories of disease. It was not until Robert Kochs 1891 proofs, for which he was awarded a Nobel Prize in 1905, viruses were confirmed as human pathogens in 1901, with the discovery of the yellow fever virus by Walter Reed. Immunology made an advance towards the end of the 19th century, through rapid developments, in the study of humoral immunity
Function of T helper cells: Antigen-presenting cells (APCs) present antigen on their Class II MHC molecules (MHC2). Helper T cells recognize these, with the help of their expression of CD4 co-receptor (CD4+). The activation of a resting helper T cell causes it to release cytokines and other stimulatory signals (green arrows) that stimulate the activity of macrophages, killer T cells and B cells, the latter producing antibodies. The stimulation of B cells and macrophages succeeds a proliferation of T helper cells.
The time-course of an immune response begins with the initial pathogen encounter, (or initial vaccination) and leads to the formation and maintenance of active immunological memory.