Adaptive immunity

Recognition and activation mechanisms

The basis of adaptive immunity consists of specialized cells — T lymphocytes and B lymphocytes. They are capable of pinpoint recognition of almost any peptide antigen due to the presence of unique antigen-recognizing receptors on their surface:

• TCR (T-cell receptor) in T lymphocytes.
• BCR (B-cell receptor) in B lymphocytes.

In the adaptive immune system, there is a strict division of functions. T-lymphocytes take responsibility for the cellular component of the immune response. B-lymphocytes, in turn, are responsible for the humoral component, carrying out the synthesis of specific antibodies. An important feature of these cells is that in order to fully activate their protective functions and transition to an active state, they must undergo an additional stage of differentiation.

Classification by formation method

Depending on how the organism receives protection, adaptive immunity is broadly divided into two forms:

1. Active immunity. Formed by the body's own forces in response to the introduction of an antigen. It can be post-infectious (arising after a naturally contracted disease) or post-vaccination (developed after targeted administration of vaccines).
2. Passive immunity. The body does not synthesize protection independently, but receives ready-made antibodies from the outside. This form can be natural, when maternal IgG class antibodies are transferred to the fetus through the placenta (transplacentally), providing protection for the newborn. There is also artificial passive immunity, which is achieved by medical administration of immune sera or ready-made immunoglobulin preparations to the patient.

Classification by presence of pathogen and scale

According to the nature of pathogen elimination (removal) from the body, immunity is:

• Sterile. Unresponsiveness persists even after the pathogen antigens have been completely removed from the body's tissues and fluids.
• Non-sterile. Protection is maintained exclusively as long as the infectious agent is physically present (persisting) in the body. A classic clinical example of such a state is immunity in tuberculosis.

When evaluating the prevalence of protection, immunity is divided into systemic (generalized), which covers the entire body as a whole, and local, in which the resistance of only individual tissues or organs increases. A key example of a local response is mucosal immunity — a complex of defense mechanisms operating at the level of mucous membranes.

Direction of the immune response

Depending on the main target of impact, immunity can be antitumor, transplantation, or anti-infective. The latter, in turn, has strict specialization according to the type of attacking pathogen:

• Antiviral
• Antibacterial
• Antifungal
• Antiprotozoal (directed against protozoan organisms)

Differences from innate immunity

While acquired immunity requires contact with an antigen and time for differentiation, innate immunity is based on basic molecular mechanisms of pathogen recognition and response to stress stimuli.

In the innate link, a leading role is played by complexly acting factors:

• Cellular mechanisms: macrophages and neutrophils, cooperating, provide capture and digestion of foreign particles (phagocytosis), as well as destruction of target cells (cytolysis).
• Humoral mechanisms: blood plasma proteins (for example, the complement system and C-reactive protein) instantly react to carbohydrate components of microorganism cell walls. Also, innate immunity cells are capable of synthesizing various active effector molecules to combat the threat.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of additional differentiation of T and B lymphocytes after meeting an antigen.
• Features and clinical significance of mucosal (local) immunity of mucous membranes.
• Humoral factors of innate immunity: the complement system and C-reactive protein.
• Principles of antitumor and transplantation immunity.

Other topics in "Immunology"

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