Development of humoral immunity in children
The synthesis of one's own antibodies develops gradually, which causes periods of vulnerability.
- IgM: Newborns possess only their own IgM, and in small amounts. Normal production is established by the end of the 1st year.
- IgG: Synthesis begins at approximately 6 months. At the same time, maternally acquired antibodies completely disappear. Full production of one's own IgG is established by 5–6 years of age.
- IgA and IgE: The capacity for their secretion matures the slowest—adult levels are reached only by age 10.
Because of this dynamic, a natural humoral immunodeficiency arises. It peaks at 6 months, when maternal IgG is already depleted and endogenous IgG is not yet produced in sufficient quantities. During this period, children are especially susceptible to infections. The condition is fully compensated by 10 years of age.
Features of cellular immunity in early childhood
The thymus-dependent arm of the immunity also does not function at full capacity after birth. At birth, the thymus mainly releases γδ T cells, which poorly recognize antigens. Fully functional αβ T cells begin to populate peripheral organs only a few days later.
Manifestations of T-cell immaturity:
- Weak response to antigens and mitogens.
- Inadequate control over humoral immunity and macrophages (mainly due to late maturation of CD4+ T helper cells).
- Low production of T-cell cytokines (IL-2, IFNγ).
Additionally, there is a delayed formation of local immunity. The lymphoid tissue of the bronchi, nasopharynx, and intestine (Peyer's patches) matures after birth with the participation of microflora. Therefore, non-specific first-line factors dominate in the mucosal defense of infants.
Thymic involution and immune senescence
Aging of the immune system begins with age-related thymic involution. The main cause lies not in the progenitor cells, but in the degradation of the organ's stroma itself.
- Loss of functions: Chemokine secretion decreases (impacting the recruitment of new cells) and thymulin production drops.
- Rate of involution: Initially, the stroma loses about 3% of its function per year, and in old age—1%. Calculated estimates show the thymus completely ceases to support T-lymphocyte maturation by 120 years of age.
As the thymus wanes, its tasks are taken over by peripheral T cells through homeostatic proliferation. With age, the proportion of memory T cells increases, while the number of naive T lymphocytes drops. In individuals over 70, the pool of CD4+ cells (especially Th1) noticeably decreases, leading to impaired tumor surveillance. At the same time, the number and function of B lymphocytes and NK cells remain normal.
Age-related changes in the humoral link and autoimmunity
Against the background of T-cell aging, humoral immunity also alters its properties. Due to weakened thymus-dependent control, the affinity (binding strength) of new antibodies decreases. However, the total concentration of serum IgG and IgA, conversely, increases due to polyclonal activation.
With age, the activity of regulatory T cells controlling tolerance to self-tissues decreases. This leads to the accumulation of autoantibodies (against DNA, collagen, thyroid gland). It is important to note that the mere presence of these antibodies in the elderly is rarely accompanied by clinical autoimmune diseases.
Briefly: answers to topical questions
Which factors of non-specific mucosal defense compensate for the delayed formation of local immunity in infants?
In the early postnatal period, compensation is provided exclusively by first-line defense factors. Chemical barriers of mucous membranes include:
- Hydrochloric acid — provides a bactericidal effect in the stomach.
- Lysozyme, lactoferrin, and free radicals — secreted by polymorphonuclear leukocytes.
- Surfactant — a protective factor of the lungs.
- Thiocyanates — secreted by the salivary glands.
- Bile acids — active in the lower gastrointestinal tract.
- Toxic low-molecular-weight fatty acids — produced by normal microflora.
Additional protection for the infant's GI mucosa is provided by breast milk factors: macrophages, neutrophils, and secretory sIgA.
Which hormones and chemokines cease to be produced during the degradation of the thymic stroma?
During age-related involution of the thymic stroma, the production of the peptide hormone thymulin significantly decreases. Furthermore, the stroma's ability to recruit lymphoid progenitor cells is impaired due to a decrease in the secretion of corresponding chemokines. Sources do not specify which exact chemokines stop being produced.
What else to study on this topic
- Mechanisms of mucosal local immunity formation
- Features of the immune response to T-independent antigens in children
- Peripheralization of thymic functions and homeostatic proliferation
- Impact of environmental factors on immune system function
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