Biological Systems MCAT Practice Exam

In T cell activation, what must occur for a T cell to respond to an antigen?

1. Binding to antigen alone

2. Simultaneous binding to antigen and MHC molecule

3. Interaction with B cells only

4. Cytokine release

The correct answer is: Simultaneous binding to antigen and MHC molecule

For a T cell to respond to an antigen, it must bind simultaneously to both the antigen and a major histocompatibility complex (MHC) molecule. This process is crucial for T cell activation. When a T cell encounters an antigen, it does so through its T cell receptor (TCR), which specifically recognizes and binds to a fragment of the antigen presented on the surface of antigen-presenting cells (APCs). The MHC molecule plays a vital role in this process by displaying the processed antigenic peptide. The interaction between the TCR and the peptide-MHC complex is essential because it provides the necessary signal to the T cell that it has encountered a relevant antigen. Additionally, this interaction is commonly strengthened through co-stimulatory signals provided by the APCs, which ensures that the T cell receives all the necessary signals for full activation. This is a hallmark of adaptive immunity, distinguishing it from innate immune responses. Other scenarios such as binding to the antigen alone, only interacting with B cells, or just cytokine release do not suffice for T cell activation because they lack the critical interaction with the MHC molecule that is necessary to properly present the antigen for T cell recognition. Therefore, the simultaneous binding to both antigen and MHC is an