Agglutination or aggregation involves the cross-linking of pathogens by antibodies to create large aggregates
Adaptive immunity is defined by two important characteristics: specificity and memory.
Specificity refers to the adaptive immune system’s ability to target specific pathogens,
memory refers to its ability to quickly respond to pathogens to which it has previously been exposed.
Specificity and memory are achieved by essentially programming certain cells involved in the immune response to respond rapidly to subsequent exposures of the pathogen
This secondary response, however, is specific to the pathogen in question.
Adaptive specific immunity involves the actions of two distinct cell types: B lymphocytes (B cells) and T lymphocytes (T cells).
their sites of maturation and their roles in adaptive immunity are very different.
B cells mature in the bone marrow and are responsible for the production of glycoproteins called antibodies, or immunoglobulins.
The maturation of T cells occurs in the thymus.
Antibodies are involved in the body’s defense against pathogens and toxins in the extracellular environment.
T cells function as the central orchestrator of both innate and adaptive immune responses. They are also responsible for destruction of cells infected with intracellular pathogens.
The targeting and destruction of intracellular pathogens by T cells is called cell-mediated immunity, or cellular immunity.
Activation of the adaptive immune defenses is triggered by pathogen-specific molecular structures called antigens.
antigens are unique to a specific pathogen.
the role of antigens is not limited to humoral immunity and the production of antibodies; antigens also play an essential role in stimulating cellular immunity, and for this reason antigens are sometimes more accurately referred to as immunogens.
antigens from bacterial cells may be associated with their capsules, cell walls, fimbriae, flagella, or pili. Bacterial antigens may also be associated with extracellular toxins and enzymes that they secrete. Viruses possess a variety of antigens associated with their capsids, envelopes, and the spike structures they use for attachment to cells.
Antigens may belong to any number of molecular classes, including carbohydrates, lipids, nucleic acids, proteins, and combinations of these molecules. Antigens of different classes vary in their ability to stimulate adaptive immune defenses as well as in the type of response they stimulate (humoral or cellular
One reason the three-dimensional complexity of antigens is so important is that antibodies and T cells do not recognize and interact with an entire antigen but with smaller exposed regions on the surface of antigens called epitopes.
A single antigen may possess several different epitopes (Figure 18.1.2 ), and different antibodies may bind to different epitopes on the same antigen
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