lecture recording immune system part 2

Adaptive Defenses

In our previous discussion, we covered innate defenses, and now we'll shift our focus to adaptive defenses, which encompass two main types: humoral and cellular immunity.

Humoral Immunity

Humoral immunity primarily operates in body fluids, or humors, and is responsible for recognizing and responding to a variety of infectious agents. This adaptive immune response is crucial; if it fails, individuals may become susceptible to diseases, including cancers. For instance, human immunodeficiency virus (HIV) cripples the immune system, leading to acquired immunodeficiency syndrome (AIDS). The effectiveness of the adaptive immune response can be amplified during inflammatory reactions, highlighting the intricate interplay among various immune components, including antibodies.

Activation and Specificity

The adaptive immune system is categorized as the third line of defense, following the skin and mucosal barriers (first line) and the innate immune response (second line). Importantly, the adaptive response is specific and systemic, meaning it targets specific pathogens and is present throughout the body. Even though it often takes longer to activate, this delayed response is vital; it ramps up while the other defenses are already engaged, leading to a coordinated attack against the invader. The hallmark of the adaptive immunity is its memory feature, which allows for a more rapid and effective response upon subsequent exposures to the same pathogen—akin to recognizing and reacting to familiar shapes like triangles.

Memory and Vaccination

Memory is a powerful tool in the adaptive immune system, leading to the effectiveness of vaccines. Vaccines work by triggering the body's immune memory, which prepares the immune system for future encounters with the pathogen. This means that after initial exposure or vaccination, subsequent encounters with the same pathogen result in a quicker and more vigorous immune response, preventing disease or reducing symptom severity.

Components of Adaptive Immunity

The adaptive immune system is composed of lymphocytes, primarily B cells and T cells.

B Cells and Humoral Immunity

B cells are responsible for producing antibodies, mediating humoral immunity. They recognize antigens and, upon activation, proliferate and differentiate into plasma cells that produce antibodies. These antibodies circulate in body fluids, playing a critical role in identifying and neutralizing pathogens. The effectiveness of these antibodies is influenced by the attributes of the antigens they encounter.

T Cells and Cellular Immunity

T cells are vital for cellular immunity. They do not produce antibodies but directly manage cellular responses against infected or aberrant cells. Both B and T cells undergo maturation processes—B cells in the bone marrow and T cells in the thymus—culminating in immunocompetence, that is, their ability to recognize specific antigens.

Antigen Recognition

The adaptive immune system's response begins with the recognition of antigens, which are distinct molecules that allow the immune system to identify foreign entities. Antigens can be classified as complete or incomplete. Complete antigens possess both immunogenicity and reactivity, stimulating the proliferation of specific lymphocytes and engaging antibodies, whereas incomplete antigens (haptens) lack one of these critical features.

The strength of an antigen is often determined by its structure; proteins, for example, are the strongest antigens due to their complexity, while smaller molecules like polysaccharides, lipids, and nucleic acids can also be recognized as complete.

Role of Antigenic Determinants

Antigens contain specific regions called antigenic determinants that facilitate the binding of antibodies. The presence of numerous antigenic determinants enhances the immune response by enabling a wider array of antibodies to bind and recognize the invader, thereby triggering a stronger immune reaction.

Positive and Negative Selection of T Cells

T cells undergo a selection process in the thymus, divided into positive and negative selection phases. Positive selection ensures that T cells can recognize self-MHC proteins while negative selection eliminates T cells that react against self-antigens. This stringent selection process results in approximately only 2% of T cells surviving, ensuring that functional T cells capable of responding to foreign invaders populate the immune system.

Passive and Active Immunity

Immunity can be classified into active and passive types. Active immunity results from the body’s own immune response to a pathogen or vaccine, allowing for the development of memory. Passive immunity, on the other hand, involves the transfer of antibodies from another source, such as from mother to child during pregnancy or via breast milk. While passive immunity offers immediate protection, it does not involve memory formation and is temporary.

Conclusion

Understanding the nuances of adaptive immunity is essential, especially in the context of vaccination, immune memory, and the mechanisms by which our bodies recognize and fight pathogens. Familiarity with these concepts not only bolsters academic knowledge but also equips future healthcare professionals to engage in informed and respectful discussions about vaccines and immune responses.