IMMUNOLOGY TEXTBOOK - Mercury Podillia 2013

STRUCTURE AND FUNCTIONAL PRINCIPLES OF THE IMMUNE SYSTEM

Critical periods of a child's immunity

First critical period — the neonatal period. This period is characterized by poor resistance to opportunistic, pyogenic, and Gram-negative microflora; a predisposition to generalized purulent-inflammatory processes and septic conditions; and high susceptibility to viral infections. Up to 0.5% of newborns show signs of congenital viral infection (Table 3).

Class="center">Table 3. Characteristics and Clinical significance of the child's immune system (Kazmirchuk V. Ye., Kovalchuk L. V., 2006)

Characteristic	Clinical significance
Imperfect Skin and mucosal barriers	Requires careful hygiene and care
Lysozyme is the most effective factor of natural resistance	One of the primary mechanisms of local defense
Incomplete phagocytosis	Exotoxin-induced systemic intoxication — high incidence of Pneumonia, complicated clinical course
Limited activity of cytokines, including interferons	Weak antiviral defense, predisposition to early generalization of viral and bacterial infections
Significantly reduced NK Cell levels	Impaired antiviral and antitumor defense
B lymphocytes exhibit low sensitivity to T-cell interleukins	Decreased Synthesis of specific Antibodies
Protection is provided solely by maternally acquired IgG	Defense against diphtheria toxin, polioviruses, measles, rubella, and microbial infections
Low production of IgA and IgM	Susceptibility to viral and bacterial infections
Relatively high IgE levels	Amplification of immune-mediated inflammation

Second critical period — 3 to 6 months of life. This period is marked by the most pronounced transient decrease in Blood immunoglobulin levels. The Immune Response is primarily of a primary nature without The formation of immunological memory. Vaccination does not establish immunological memory, and only revaccination elicits a secondary immune response. T-cell immunodeficiencies manifest by 4 to 5 months of age, whereas antibody deficiencies typically present around 6 months of age (Table 4).

Table 4. Characteristics and clinical Significance of the child's immune system during the second critical period

Characteristic	Clinical significance
Substantial decrease in IgG (due to the Catabolism of maternally acquired antibodies)	Reduction in passive humoral Immunity
Increased synthesis of sIgA starting from 3 months of age, though local immune deficiency persists up to 4 years	High susceptibility to acute respiratory viral infections (ARVI)
Lowest levels of all immunoglobulin classes	Physiological hypogammaglobulinemia
Reduced capacity for interferon synthesis	Frequent ARVI
Primary immune response with IgM synthesis develops against most Antigens, lacking immunological memory	Atypical clinical course of measles and pertussis, leaving no lasting immunity; hepatitis B virus rarely causes jaundice

Third critical period — 2 to 3 years of life. A significant expansion of the child's social contacts leads to an increased frequency of infectious diseases, resulting in the decompensation of immature immune mechanisms and the manifestation of immune abnormalities (Table 5).

Table 5. Characteristics and clinical significance of the child's immune system during the third critical period

Characteristic	Clinical significance
Persistence of the primary type of immune response	Poor adaptation to nursery or kindergarten environments
Ongoing IgG deficiency	Continued susceptibility to VIRAL INFECTIONS AND Haemophilus influenzae; maturation of humoral immunity
Increased sensitivity of B lymphocytes to interleukins, activation of helper function	Emergence of immune system abnormalities
Immaturity of mucosal immune processes	Children are susceptible to viral infections, frequent ENT disorders

Fourth critical period — 4 to 6 years of life. The establishment of adaptive immunity nears completion. Upper Respiratory Tract infections tend to become chronic or recurrent due to local immune deficiency (Table 6).

Table 6. Characteristics and clinical significance of the child's immune system during the fourth critical period

Characteristic	Clinical significance
Occurrence of the second blood cross-over: absolute lymphocyte count decreases while neutrophil count increases	Poor adaptation to children's groups
Formation of a secondary Immune Response to most antigens	Increase in the immunoregulatory index; decrease in the absolute number of B lymphocytes; IgM reaches adult levels
Secretory IgA remains significantly below adult levels	Persistent mucosal immune deficiency
Elevated IgE levels	Increased frequency of immunodeficiency manifestations

Fifth critical period — 12 to 13 years of life. This period is marked by the onset of active gonadal function, resulting in gender-specific differences in immune status (Table 7).

Table 7. Characteristics and clinical significance of the child's immune system during the fifth critical period

Characteristic	Clinical significance
Reduction in the mass of Lymphoid Organs	Poor adaptation to peer groups
In boys, stimulation of sex hormone (androgen) secretion, which suppresses immune function	Increased susceptibility to Mycobacterium tuberculosis
In girls, an unbalanced estrogen-to-progesterone ratio leads to decreased T-cell suppressor function	More severe clinical course of allergic and autoimmune diseases

Last update: 13/08/2026

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