Final answer:
The MHC Class II is made up of two chains, the α and β chains, each with two domains. These chains assemble to form an antigen-binding cleft, critical for the immune response, and are found on antigen-presenting cells such as macrophages, dendritic cells, and B cells.
Step-by-step explanation:
The MHC Class II structure is composed of two chains, an α (alpha) and a β (beta) chain, which are approximately similar in length. Each of these chains has two domains, resulting in the chains folding into separate domains named α₁, α₂ for the alpha chain and β₁, β₂ for the beta chain. The antigen-binding cleft of MHC Class II is specifically formed by the α₁ and β₁ domains. This cleft is where abnormal or non-self-antigens are presented to T cells, which is essential for the activation of the immune response.
MHC Class II molecules are primarily found on macrophages, dendritic cells, and B cells. These cells are known as antigen-presenting cells which play a critical role in adaptive immunity, presenting antigens to T cells that express CD4 molecules on their surface.