[7] Immunology has applications in numerous disciplines of medicine, particularly in the fields of organ transplantation, oncology, rheumatology, virology, bacteriology, parasitology, psychiatry, and dermatology.

Ehrlich interpreted this as immunization and observed that it was abruptly initiated after a few days and was still in existence after several months.

However, many components of the immune system are cellular in nature, and not associated with specific organs, but rather embedded or circulating in various tissues located throughout the body.

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.

The study of the molecular and cellular components that comprise the immune system, including their function and interaction, is the central science of immunology.

It is now getting clear that the immune responses contribute to the development of many common disorders not traditionally viewed as immunologic,[15] including metabolic, cardiovascular, cancer, and neurodegenerative conditions like Alzheimer's disease.

Besides, there are direct implications of the immune system in the infectious diseases (tuberculosis, malaria, hepatitis, pneumonia, dysentery, and helminth infestations) as well.

Antibodies specific for a desired antigen can be conjugated with an isotopic (radio) or fluorescent label or with a color-forming enzyme in order to detect it.

Immunotherapy is most commonly used to treat allergies, autoimmune disorders such as Crohn's disease, Hashimoto's thyroiditis and rheumatoid arthritis, and certain cancers.

It also involves diseases of other systems, where immune reactions play a part in the pathology and clinical features.

Clinical immunologists also study ways to prevent the immune system's attempts to destroy allografts (transplant rejection).

As part of their training fellows may do additional rotations in rheumatology, pulmonology, otorhinolaryngology, dermatology and the immunologic lab.

[18] When health conditions worsen to emergency status, portions of immune system organs, including the thymus, spleen, bone marrow, lymph nodes, and other lymphatic tissues, can be surgically excised for examination while patients are still alive.

[21][22][23] In the mid-1950s, Macfarlane Burnet, inspired by a suggestion made by Niels Jerne,[24] formulated the clonal selection theory (CST) of immunity.

Once born, a child's immune system responds favorably to protein antigens while not as well to glycoproteins and polysaccharides.

This is due to lower opsonic activity, as well as diminished up-regulation of integrin and selectin receptors, which limit the ability of neutrophils to interact with adhesion molecules in the endothelium.

Although, the number of total lymphocytes is significantly higher than in adults, the cellular and humoral immunity is also impaired.

[41] Between six and nine months after birth, a child's immune system begins to respond more strongly to glycoproteins, but there is usually no marked improvement in their response to polysaccharides until they are at least one year old.

[43][44] During adolescence, the human body undergoes various physical, physiological and immunological changes triggered and mediated by hormones, of which the most significant in females is 17-β-estradiol (an estrogen) and, in males, is testosterone.

[51] Ecoimmunology, or ecological immunology, explores the relationship between the immune system of an organism and its social, biotic and abiotic environment.

[52] Behavioural immunity, a phrase coined by Mark Schaller, specifically refers to psychological pathogen avoidance drivers, such as disgust aroused by stimuli encountered around pathogen-infected individuals, such as the smell of vomit.

[54] This indicates that laying eggs on toxic plants is a costly behaviour in Monarchs which has probably evolved to reduce the severity of parasite infection.