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In the immune system, T cells play a crucial role in identifying and fighting against pathogens like viruses and bacteria. T cells activation is a multi-step process beginning with the recognition of antigens. This starts when antigen-presenting cells (APCs), such as dendritic cells or macrophages, present processed antigen fragments on their surface. However, simply recognizing an antigen is not enough to activate a T cell. The antigen must be presented in a specific context, using proteins called the Major Histocompatibility Complex (MHC).

Activation occurs when a T cell's receptor (TCR) binds to an antigen presented on the surface of an APC only if it is paired with the correct MHC molecule. This binding initiates a cascade of reactions inside the T cell, leading to its activation. The activated T cell then undergoes proliferation, creating numerous T cell copies that differentiate into specialized types, such as helper T cells or cytotoxic T cells. This ensures a robust immune response against the pathogen.

• Activation depends on the binding of T cell receptor and antigen with MHC.
• Once activated, T cells multiply and differentiate.
• Different types of T cells serve specific roles in immunity.

Antigen-presenting cells (APCs) are key players in the immune system, serving the essential function of showing processed foreign molecules (antigens) to T cells, which starts the adaptive immune response. The most common types of APCs include dendritic cells, macrophages, and B cells. These cells actively phagocytize, or "eat," pathogens such as bacteria and viruses. After ingesting these pathogens, they process them into smaller fragments, making them suitable for presentation to T cells.

The job of APCs doesn’t stop there. They must also ensure that these antigen fragments are correctly displayed on their cell surface to effectively communicate with T cells. This is done by inserting the fragments into their cell membrane, a process supported by Major Histocompatibility Complex (MHC) molecules. When the antigen and MHC molecule form a complex on the APC surface, it serves as a key signal for T cells. Therefore, APCs act as a crucial bridge between innate and adaptive immunity, ushering in a more sophisticated immune reaction.

• APCs include dendritic cells, macrophages, and B cells.
• They process and present antigens on their surface.
• APCs use MHC to display antigens for T cell recognition.

The Major Histocompatibility Complex (MHC) is a set of proteins found on the surfaces of cells that are essential for the adaptive immune system to recognize foreign molecules. These complex proteins ensure that T cells can properly identify antigens and launch an appropriate immune response. MHC molecules come in two main classes: MHC Class I and MHC Class II, each involved in presenting different types of antigens.

MHC Class I molecules are present on nearly all nucleated cells, presenting endogenous antigens, typically originating from intracellular pathogens like viruses, to cytotoxic T cells. On the other hand, MHC Class II molecules are primarily expressed on professional antigen-presenting cells such as macrophages, dendritic cells, and B cells. They present exogenous antigens, sourced from pathogens outside the cell, to helper T cells.

• MHC is crucial for antigen presentation.
• Two classes: MHC I (present in most cells) and MHC II (restricted to APCs).
• MHC helps T cells differentiate self from non-self antigens.