An antigen is any substance that can trigger an immune response in the body. This substance is typically a protein or carbohydrate, but can also be a larger molecule like a polysaccharide. The immune system recognizes antigens as foreign or potentially harmful, prompting a defensive reaction.

Antigens play a crucial role in the immune system by acting as signals that alert the body to the presence of pathogens (e.g., bacteria, viruses, fungi, parasites) or other foreign invaders. When an antigen is detected, it triggers the activation of immune cells, such as B lymphocytes (which produce antibodies) and T lymphocytes (which directly attack infected cells or coordinate the immune response). Antibodies bind specifically to the antigen, marking it for destruction or neutralizing its effects. This process is fundamental to adaptive immunity, allowing the body to develop a targeted and long-lasting defense against specific pathogens.

Self antigens are molecules that are naturally present in the body's own cells and tissues. They are a normal part of the body's composition and are not recognized as foreign. Non-self antigens, on the other hand, are molecules that originate from outside the body, such as those found on pathogens (bacteria, viruses, fungi, parasites) or foreign substances (e.g., pollen, transplanted tissues).

The immune system *must* be able to distinguish between self and non-self antigens to avoid attacking the body's own tissues. If the immune system mistakenly attacks self antigens, it leads to autoimmune diseases, where the body's immune response is directed against its own cells and organs. This can cause a wide range of health problems, from rheumatoid arthritis to type 1 diabetes. Therefore, a critical function of the immune system is to develop mechanisms to tolerate self antigens and mount an effective response only against non-self antigens.

In a primary viral infection, the initial response relies heavily on innate immunity, with macrophages playing a key role in early pathogen recognition and containment. Macrophages phagocytose the virus and present viral antigens on MHC class II molecules to helper T-cells (Th cells). This antigen presentation is critical for initiating the adaptive immune response.

Activated Th cells release cytokines that stimulate B-lymphocytes to differentiate into plasma cells. Plasma cells then produce antibodies specific to the viral antigens. These antibodies can neutralize the virus by preventing it from entering host cells, or they can opsonize the virus, making it easier for phagocytes to engulf and destroy. Antibody production is a hallmark of the humoral immune response in viral infections.

Cytotoxic T-cells (killer T-cells) are activated if they recognize viral antigens presented on MHC class I molecules on infected host cells. These killer T-cells then directly kill the infected cells, preventing viral replication and spread. This is an example of cell-mediated immunity. The interaction between Th cells and killer T-cells is essential for a coordinated and effective response to viral infection.

The primary immune response to a viral infection is relatively slow, taking several days to fully develop. However, it generates a population of memory B-cells and memory T-cells, which provide long-lasting immunity against subsequent viral infections. The initial response is crucial for controlling the viral load and preventing severe disease.