Adaptive immunity

Three main features of adaptive immunity

The adaptive immune response is based on three fundamental characteristics that fundamentally distinguish it from the body's nonspecific defense reactions:

1. Specificity. The infectious process stimulates the production of defense mechanisms directed strictly against the specific pathogen that caused the disease. In some cases, defense may also act against a very closely related agent, but it is never universal.
2. Immunological memory. After the body has successfully mounted a response to a specific pathogen, the system "remembers" it. The mechanism of this memory lies in the unique ability of cells to reproduce the biochemical composition of the microorganism's antigenic determinants. This memory usually persists throughout a person's life. It forms the basis of immunization—both natural (following a disease) and artificial. If re-infection with the same pathogen occurs, a secondary immune response is triggered, developing significantly faster and proceeding much more intensely than the primary one.
3. Distinguishing "self" from "non-self". The immune system possesses a subtle recognition mechanism for structures of its own tissues and foreign agents. A healthy body maintains immunological tolerance—a state of stable and specific unresponsiveness to its own tissues. If this mechanism fails and the body mistakenly perceives its own cells as foreign, a severe pathology develops—an autoimmune response.

Forms of the specific response

The specific immune response is triggered by pathogen antigens and manifests in two main forms. Normally, they do not work in isolation, but closely interact through the cooperation of lymphocytes, macrophages, and other cells.

• Humoral immune response. B lymphocytes (B cells) are responsible for this form. Upon contact with an antigen, they initiate a process resulting in the formation of effector cells—plasma cells. These cells actively synthesize specific antibodies (Abs). These antibodies circulate in body fluids and neutralize the very antigen that initially triggered their synthesis. Based on the humoral response, passive immunity can be created: it is formed artificially when a healthy individual is injected with blood serum from an immune donor that already contains pre-formed antibodies.
• Cellular immunity. This form is mediated by T lymphocytes (T cells) and other immunocompetent cells. The effector cells here are cytotoxic T lymphocytes (CTLs), as well as specialized T-cell populations involved in delayed-type hypersensitivity (DTH) reactions.

The most important outcome of the clonal proliferation of both T and B cells is the formation of antigen-specific populations—memory cells. Their primary function is to provide a rapid secondary response to re-exposure to the same pathogen.

Where do immune processes take place?

Recognition of foreign antigens and the direct functioning of cellular mechanisms of adaptive immunity occur not randomly, but in strictly defined structures—the secondary lymphoid organs. These include:

• Lymph nodes;
• Spleen;
• Palatine tonsils;
• Solitary follicles (so-called Peyer's patches) located in the small intestine;
• The lymphoid apparatus of the vermiform appendix.

Although these organs are scattered throughout the body, various parts of the lymphoid system function as a single entity. This unity of the system is ensured by the process of lymphocyte recirculation—the continuous exchange of cells between all compartments. The main goal of this continuous recirculation is to maximize the probability of contact between antigen-specific lymphocytes and the antigen determinants of the pathogen, wherever it may appear in the body.

Factors influencing the immune response

Even with established immunological memory and coordinated functioning of all links of the lymphoid system, the ultimate strength and character of the specific immune response may vary. According to the general principles of immune system organization, the severity of the reaction is influenced by the following key factors:

1. Nature of the antigen (its structure and origin).
2. Dose of the antigen (the amount of pathogen that entered the body).
3. Route of entry (via respiratory tract, digestive tract, blood, etc.).
4. Genetic constitution of the individual (hereditary features of a particular person's immune system).

Class and exam questions

What else to study on this topic

• Mechanisms of cooperation between T lymphocytes, B lymphocytes, and macrophages
• Processes of clonal proliferation and differentiation of lymphocytes
• Pathogenesis of autoimmune diseases upon loss of tolerance
• The role of T cells in the development of delayed-type hypersensitivity (DTH)
• Influence of genetic constitution and antigen dose on response magnitude

Other topics in the section «Immunopathological processes»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.