Immune deficiencies in second child?
Since maternal stores take time to recover from pregnancy, birth, and lactation, maybe it could effect milk supply for the next baby. Six months or more after weaning is often reccomended before becoming pregnant, so that mother can rebuild her maternal stores, then 3-5 years between births. I can understand your curiosity in this matter but I think it is great that you are breastfeeding and giving your baby the over all immune advantage of breastmilk. The info below may be of help (or not).
I've been interested in this myself, including the examination of how a mother feels in pregnancy depending on how close the kids are or how well her nutrient stores were replenished through diet, before getting pregnant again. Pregnancy and nursing is a huge drain of nutrients for the mother, and it's hard work to restore oneself to have enough to "give" to another baby. You could improve both you and your son through adding nutrient dense foods to your diet, such as quality full fat dairy, cod liver oil, plenty of fat, organ meats (or organ pills if you don't like taste), pastured eggs, and more. If interested, you could post on the Traditional Foods board for some other suggestions.