Immunity and the immune system starts its development much before birth in the fetus and this development continues even after reaching adulthood.

The ability to respond to antigens and foreign invaders is thus dependent on:

Age of the person

Immunity is developing and still to reach maturity in babies and in children. In addition, with age the immune system loses some of its earlier vigour. Thus both extremes of age are relatively immune-deficient and prone to infections and other invasions of the immune system.

Neonates or newborns are said to be in a state of physiological immunodeficiency, because both their innate and adaptive immunological responses are greatly suppressed. After birth, a child’s immune system responds favorably to protein antigens while not as well to glycoproteins and polysaccharies.

Many of the infections acquired by neonates or newborns thus are caused by low virulence organisms like Staphylococcus and Pseudomonas. In these babies opsonic activity and the ability to activate the complement cascade is very limited.

Phagocytic activity is also greatly impaired in newborns. This is due to lower opsonic activity, as well as diminished up-regulation of integrin and selectin receptors. This prevents the neutrophils from interacting with adhesion molecules in the endothelium. Newborns also have slow monocytes and have a reduced ATP production, which also limits the newborns phagocitic activity.

The cellular and humoral immunity is also impaired. Antigen presenting cells in newborns have a reduced capability to activate T cells. Also, T cells of a newborn proliferate poorly. B cells develop early in pregnancy while the baby is within the womb but are not fully active.

Adolescents undergo several physical, physiological and immunological changes – this is mediated by different hormones. Depending on the sex, either testosterone or 17-β-oestradiol, act on male and female bodies respectively around this age. There is evidence that these steroids act directly on the primary and secondary sexual characteristics and also have effects on the development and regulation of the immune system. Cell surface receptors on B cells and macrophages may detect sex hormones in the system and autoimmunity and related disorders may begin around this age as well.

17-β-oestradiol has been shown to regulate the level of immunological response and testosterone suppresses the stress response to infection. Another androgen like DHEA has the opposite effect, as it increases the immune response. Puberty also affects the involution of the Thymus and thus affects the immunological response of the individual.

Type of antigen

Some of the antigens are handled well by the first line of defence or the innate immunity. When the innate immunity is penetrated or overwhelmed, the adaptive or humoral immunity steps in. Antigen type determines the chances of success or failure to overcome the invasion.

Maternal factors

Maternal factors that influence effect of an antigen. At birth, the immunoglobulin present in the baby is maternal IgG. Other immunoglobulins do not cross the placenta so maternal IgM, IgD, IgE and IgA are not found in the baby. Some IgA is provided in breast milk. The mother may pass on some of her antibodies to the baby and this may protect the newborn for a few months after birth. For example, the baby is usually protected against whooping cough for the first six months of life due to presence of maternal antibodies.

Area of body

The area of the antibody where the antigen is presented.

Further Reading

• All Immunology Content
• What is Immunology?
• Classical Immunology
• Clinical Immunology
• Evolutionary Immunology

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.