Various factors cause secondary immunodeficiencies

In this article, I briefly describe various factors causing secondary immunodeficiencies.

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Secondary immunodeficiencies

Inherited genetic defects give rise to primary immunodeficiency whereas secondary immunodeficiency is acquired. The loss of immune function results from exposure to an external agent. These external agents include many diseases and infections, medical treatments with immunosuppressive drugs, and social conditions that cause malnutrition. Acquired immunodeficiency syndrome (AIDS) is the best-known secondary immunodeficiency, which results from infections with the human immunodeficiency virus (HIV) (figure 1). The symptoms of secondary immunodeficiency include heightened susceptibility to common infectious agents, and opportunistic infections, that are absent in a healthy immune system.

The factors causing secondary immunodeficiency

Many factors cause secondary immunodeficiency. It can be agent-induced or due to natural factors affecting immune function.

Agent-induced immunodeficiency

This kind of immunodeficiency results from exposure to any environmental agent that induces an immunosuppressed state. Patients after organ transplantation use corticosteroids to prevent rejection. Corticosteroids also help to combat autoimmune diseases like rheumatoid arthritis. These immunosuppressive drugs differ in their mechanism of action and specifically target T cells or B cells. Many cytotoxic drugs or radiation treatments used in the treatment of cancer, frequently damage rapidly dividing cells in the body, including those of the immune system. This induces a state of temporary immunodeficiency. Patients undergoing such therapy must be closely monitored and treated with antibiotics and/or immunoglobulin on the occurrence of infection.

Acquired hypogammaglobulinemia- a secondary immunodeficiency

After birth, certain acquired factors cause acquired hypogammaglobulinemia. Low levels of antibodies in the blood signal the presence of this condition. The disease is typically seen in young adults with very low total immunoglobulin. The B cells and T cells are present in normal amounts. Recurrent infections are the primary symptoms of the disease. This condition is generally treated by immunoglobulin therapy, which allows people to lead a normal life. There is no proof of genetic transmission of the disease. Mothers with acquired hypogammaglobulinemia deliver normal infants. However, at birth, a scarcity of circulating immunoglobulin in these infants is seen. This is because of the lack of IgG in the maternal circulation that can be passively transferred to the infant.

Extremes of age- A natural factor causing secondary immunodeficiency

Individuals suffer from immunodeficiencies at a very young age or in an elderly state. Neonates, particularly premature infants, are highly vulnerable to infections, with the severity of immune dysfunction correlating with the degree of prematurity. While full-term, healthy newborns possess all fundamental immune components, their innate and adaptive immune functions require time to develop completely. Passive maternal antibodies provide some protection for approximately the first six months of life, contributing to the rationale behind a gradual vaccination schedule for common childhood infectious diseases, typically recommended between 2 and 15 months of age. As individuals age, their susceptibility to infections—especially from bacteria and viruses— malignancies, increases. Cell-mediated immunity declines, characterized by a reduced diversity of T cells. Although memory B cells and circulating IgG levels rise, the overall diversity of the B-cell repertoire diminishes.

Malnutrition- A cause of acquired immunodeficiency

Severe malnutrition is the most common cause of acquired immunodeficiency, affecting innate and adaptive immunity. Prolonged periods of very low protein-calorie intake are linked to a decline in the number and function of T cells, whereas negative effects on B cells may take longer to manifest. Some research suggests that limited protein availability may shift the immune system toward anti-inflammatory pathways. Beyond protein, deficiencies in essential micronutrients like zinc and vitamin C likely contribute to overall immune weakness and an increased risk of opportunistic infections associated with malnutrition. This condition can be worsened by stress and infections, which may lead to diarrhea, further impairing nutrient absorption in the gut. Additionally, vitamin D deficiency, crucial for calcium absorption and bone health, has been associated with a reduced capacity of macrophages to combat intracellular pathogens such as Mycobacterium tuberculosis, which is prevalent in many regions where malnutrition is most widespread.

Conclusion

Secondary immunodeficiency is the loss of immune function from exposure to an external agent. These external agents include many diseases and infections, medical treatments with immunosuppressive drugs, and social conditions that cause malnutrition. The symptoms of secondary immunodeficiency include heightened susceptibility to common infectious agents, and opportunistic infections, that are absent in a healthy immune system.

Many factors cause secondary immunodeficiency. It can be agent-induced or due to natural factors affecting immune function. Agent-induced immunodeficiency results from exposure to any environmental agent that induces an immunosuppressed state.