Cellular and molecular interactions determining whether antigen signals the activation or inactivation of CD4 T cells

免疫系统存在于所有脊椎动物中，为抵御入侵者提供了复杂的防御。它有一个显著的特性，能够以一种非常特殊的方式对抗这些入侵者，比如，对一种病毒的免疫反应，只对这种病毒有效，对另一种病毒无效。这就是特异性。免疫系统的识别分子与入侵者的相互作用通常被比作钥匙与锁的相互作用。其次，免疫系统似乎能够抵抗它遇到的每一个入侵者。这个性质叫做通用性。特殊性和普遍性可以比作一个钥匙店，手里有钥匙，可以打开世界上每一把锁，甚至那些还没有发明的锁！能够被这些特定分子识别的分子被称为抗原。

The immune system exists in all vertebrates and provides a sophisticated defense against invaders. It has the remarkable property of being able to fight these invaders in a very specific way, such that the immune response to one virus, say, is only effective against this virus and not another virus. This is the property of specificity. The interaction of the recognition molecules of the immune system and the invader is often likened to the interaction of a key with its lock. Secondly, the immune system appears to be able to fight every invader it encounters. This property is called universality. Specificity and universality can be likened to a key shop, that has keys on hand able to open every lock in the world, even those not yet invented! Molecules able to be recognized by these specific molecules are called antigens.