Immunity

The question asks us to identify which of the given options is involved in the innate immune response in higher mammals. Innate immunity refers to the non-specific first line of defense that an organism mounts against pathogens. It is the body's initial protective mechanism as opposed to adaptive immunity, which is more specialized and specific.

Let's evaluate each of the options provided:

• T cell antigen receptor: T cell antigen receptors are part of the adaptive immune system. They are specific to particular antigens and are not involved in the innate immune response.
• B cell antigen receptor: Like T cell receptors, B cell antigen receptors are also part of the adaptive immune system. They play a role in recognizing specific antigens to mount an appropriate immune response.
• Toll-like receptor: Toll-like receptors (TLRs) are a class of proteins that play a crucial role in the innate immune system. They recognize structurally conserved molecules derived from microbes, allowing the immune system to detect the presence of pathogens generally and mount a quick response.
• Major histocompatibility complex-II molecule: MHC-II molecules are involved in the adaptive immune system. They present processed antigen fragments to helper T cells, facilitating the specific immune response.

Therefore, the correct answer is the Toll-like receptor, as it is directly involved in the innate immune response.