Specific body defence, known as adaptive or acquired immunity, targets particular pathogens with high precision. Unlike innate immunity which acts generally, adaptive immunity tailors its response to specific foreign substances. It develops after exposure to an infection or vaccination and provides long-lasting protection through immunological memory.
Two main branches comprise this system:
An antigen is any substance that the immune system recognizes as foreign and is capable of triggering an immune response. While the terms antigen and immunogen are often used interchangeably, an immunogen specifically induces an immune response, whereas some small molecules called haptens only become immunogenic when bound to carrier proteins.
Antigens typically possess specific regions on their surface known as epitopes or antigenic determinants. These are the exact molecular sites that immune receptors and antibodies bind to. Pathogens carry multiple distinct epitopes, allowing the immune system to mount diverse responses against a single invading organism.
Lymphoid organs are categorized into primary and secondary structures where immune cells form, mature, and interact with antigens.
White blood cells driving these responses are primarily lymphocytes, including B-cells, T-cells, and natural killer cells, supported by antigen-presenting cells like macrophages and dendritic cells.
Humoral immunity revolves around the action of B-lymphocytes. When a naive B-cell encounters its specific antigen in a secondary lymphoid organ, it binds to the antigen and internalizes it. With help from activated helper T-cells, the B-cell undergoes clonal expansion, multiplying rapidly.
Most of these cloned cells differentiate into plasma cells, which act as factories pumping large volumes of specific antibodies into the blood and lymph. A small portion of the clones become memory B-cells, remaining dormant in the body to ensure a rapid response if the same pathogen invades again in the future.
Cell-mediated immunity defends the body against intracellular pathogens, such as viruses and certain bacteria, as well as cancer cells. This branch relies on T-lymphocytes, which do not recognize free antigens. Instead, antigen-presenting cells must process and display antigen fragments on their surface using major histocompatibility complex (MHC) molecules.
Two main functional types of T-cells drive this response:
Antibodies, also known as immunoglobulins (Igs), are Y-shaped proteins produced by plasma cells. Each antibody molecule consists of two identical heavy chains and two identical light chains held together by disulfide bonds. The tips of the Y-shape form the antigen-binding site (Fab region), which varies greatly between different antibodies to match unique epitopes. The stem of the Y (Fc region) determines the antibody class and interacts with immune cells and complement proteins.
Antibodies protect the body through several mechanisms:
Adaptive immunity is acquired through natural or artificial means, categorized as either active or passive.
Immunological memory is the hallmark of adaptive immunity. The primary immune response following a first exposure to an antigen is relatively slow and produces a moderate amount of antibodies. However, subsequent exposures trigger a secondary immune response that is much faster, stronger, and lasts longer due to the presence of long-lived memory T and B cells.
Vaccinations exploit this principle by introducing harmless antigens, such as killed or weakened pathogens, or pieces of their protein coats. This primes the immune system to create memory cells without causing the actual disease, ensuring rapid defense if exposed to the real pathogen later.