Final answer:
The immune system uses T cell-independent and T cell-dependent mechanisms to recognize and react to antigens. T cell-independent antigens activate B cells. The correct option is D.
Step-by-step explanation:
The immune system recognizes pathogens through various mechanisms, including T cell-dependent and T cell-independent antigens. A T cell-independent antigen can activate B cells without the assistance of helper T cells. These antigens are typically non-protein compounds such as polysaccharides or lipopolysaccharides that have repetitive structures, allowing for the crosslinking of B cell receptors (BCRs).
In contrast, T cell-dependent antigens do not exhibit the same degree of repetition and cannot crosslink BCRs as effectively. B cells require two signals to get activated in response to these antigens: recognition of the antigen by surface immunoglobulin and assistance from helper T cells (Th2 cells) that provide cytokines after interacting with presented antigens on class II MHC molecules on the B cell.
In summary, T cell-independent activation of B cells relies on the structural repetition of antigens on pathogens, while T cell-dependent activation requires additional signals from helper T cells. Plasma cells, which are differentiated B cells, are the effector units that secrete antibodies into the bloodstream to bind to specific antigens and mark them for destruction. The correct option is D.
The question is incomplete, The complete question is given below:
TI (thymus Independent) Ags are Ags that do not require the B cells to be activated by CD4 T cells. Example of classic TI-1 Ag (thymus Independent) is the LPS gram negative bacteria. A classic example of TI-2 Ags is the cell wall polysaccharides of the pneumococcus (streptococcus pneumonia). TI-2 Ags have repeating epitopes in high concentration density in the surface of the pathogen. This extensive cross linking that occurs negates the need for additional signals for B cell activation.
A) Classic TI-1 Ags include the cell wall polysaccharides of pneumococcus.
B) Classic TI-2 Ags have repeating epitopes with low concentration density.
C) LPS from gram-negative bacteria is an example of a Thymus-Dependent Antigen.
D) TI-2 Ags negate the need for additional signals for B cell activation due to extensive cross-linking.
Select the option that accurately represents the characteristics of Thymus-Independent Antigens as described in the passage."