Acute and chronic alcohol exposure suppresses all branches of the immune system, including early responses to infection and the tumor surveillance system, e.g. :

• Decrease in the ability to recruit and activate germ-killing white blood cells.
• Increased incidence of breast cancer in people who consume alcohol.

Alcohol also impairs the ability of neutrophils to migrate to sites of injury and infection (chemotaxis). In addition, it has been observed that human and animal cells isolated after administering alcohol showed an altered production of macrophages and cytokines. Some experts suspect that alcohol exerts an "all-or-none" effect on immune response; that is, the presence or absence of alcohol, rather than its amount dictates the immune response. Chronic exposure to alcohol alters the production of different hormones with immunomodulatory capacity. Hence alcohol effects on immunity could involve an indirect mechanism in which alcohol alters hormone levels and, in turn, the hormones regulate immune responses.
Taking all of these data into account, alcohol is clearly not recommended in athletes because it can increase their susceptibility to infections and other nosologies above all in situations of exercise-induced stress.