Most guys (at least in the U.S.) love the taste of meat and grew up on a steady diet of it. As a kid, I had meat EVERY lunch and dinner and milk for breakfast with almost no exceptions. What few people realize, though, is that a significant body of research shows that protein can likely hampers immune function.
It all started with an abundance of animal research that showed that higher protein diets, usually in the 15-25% range, left animals much more vulnerable to infections, parasites and the like. For example, mice fed either a high protein (25%) or low protein (12.5%) diet were infected with malaria by researchers.  All the mice on the higher protein died and all the mice on the lower protein diet survived. The researchers explained that this was primarily due to an improved antibody response with lower levels of dietary protein.
Other research has uncovered similar results. Guinea pigs were infected with peritonitis-inducing bacteria and given either 5%, 10%, 15% or 20% protein diets.  As you might have guessed, the lower the protein, the more animals that survived. In fact, researchers often gave animals a low protein diet in order to boost their immune systems during research. 
So has all of this translated to compromised immune function in studies on humans? Two studies on women have shown significantly greater risks of various autoimmune disorders with higher protein diets. One study of middle-aged French women found that those eating the most protein, especially from meat sources, were three times as likely to develop Irritable Bowel Syndrome.  And one study on Connecticut women showed a significant increase in non-Hodgkin’s lymphoma from higher levels of protein. 
The bottom line is that higher protein diets, such as is common in modern societies and in bodybuilding circles – and I eat a lot of protein myself in the form of egg whites and undenatured rice protein – may often compromise immune function. Again, see my link on Immunity for legitimate ways to optimize your protection against the onslaught of bacteria and viruses around us.
NOTE: Levels that are too low in protein can definitely whack immunity. Plus, not all proteins are necessarily created equal: whey actually has a very positive effect on immunity, due to its glutathione-increasing and other important properties. (Be sure that you purchase undenatured or biologically active whey, which I document in my link called Yes Whey. ) Cassein, the primary milk protein, is a “mixed bag”, sometimes enhancing, sometimes whacking immune function. I should also mention that some studies on younger animals show a higher protein diet actually boost immunity. 
1) J Parasitol, 2006 Jun, 92(3):531-8, “Protection against malaria due to innate immunity enhanced by low-protein diet”
2) Ann Surg, 1989 April, 209(4):448 454, “Low protein diets improve survival from peritonitis in guinea pigs”
3) International Journal for Parasitology, Jul 1993, 23(4):471-476, “The effect of age and dietary protein on immunity and resistance in lambs vaccinated with Trichostrongylus colubriformis”
4) J Clin Microbiol, 1991 July, 29(7): 1317-1322, “Model of recurrent pulmonary aspergillosis in rats”
5) Am J Gastroenterol, advance online publication 11 May 2010, “Protein Intake and Risk of Inflammatory Bowel Disease: The E3N Prospective Study”
6) American Journal of Epidemiology, 2011, 159(5):454-466, “Diet and Nutrient Intakes and Risk of Non-Hodgkin s Lymphoma in Connecticut Women”