The immune system is a complex network of cells and molecules that protects the body from foreign invaders. T cells, a type of white blood cell, play a crucial role in this process. They recognize and attack foreign substances, such as pathogens, that enter the body.

T cells require the help of antigen-presenting cells (APCs), which process foreign antigens into smaller fragments that can be recognized by T cells. These APCs are highly specialized cells that efficiently internalize antigens through phagocytosis or receptor-mediated endocytosis.

Dendritic cells, a type of professional APC, are particularly efficient at antigen processing and presentation. They are highly mobile and can be found in tissues first exposed to foreign antigens, such as the skin, mucosal surfaces, and lymphoid organs.

Macrophages may also function as APCs, and are found in all tissues. They can engulf and process foreign antigens and release cytokines, which activate T cells and other immune cells, thereby coordinating the immune response.

B cells are another type of APC that can present antigens to T cells. When an antigen is presented to a B cell, it activates the B cell to produce antibodies against the antigen. This activation process involves the binding of the antigen to the B cell receptor, followed by a series of molecular signaling events that produce antibodies.

When an APC engulfs an antigen, it breaks into smaller fragments and combines them with major histocompatibility complex (MHC) molecules. The antigen-MHC complex is then transported to the APC's plasma membrane, where it is presented to a T cell.

The appropriate T cell receptor recognizes the processed antigen complex, triggering an immune response against the foreign invader. This process is crucial for protecting the body from infection and disease.