伤寒和副伤寒是由沙门氏菌引起的传染病，它们在临床上表现出许多相似的症状。其中一个显著的特点是白细胞数量的减少。那么，为什么在伤寒和副伤寒时会出现白细胞减少呢？

首先，我们需要了解白细胞在人体免疫系统中的作用。白细胞是一种重要的免疫细胞，主要负责抵御病原体的侵袭。当我们感染细菌或病毒时，白细胞会迅速增加以应对感染。然而，在伤寒和副伤寒这种疾病中，白细胞数量却会减少。

研究表明，白细胞减少可能与沙门氏菌的生物学特性有关。沙门氏菌是一种革兰阴性杆菌，具有强烈的侵袭性。当沙门氏菌进入人体后，它会通过细胞内寄生来抵抗宿主免疫系统的攻击。这种寄生机制使沙门氏菌能够逃避宿主的免疫应答，从而引起感染。

此外，沙门氏菌还能干扰白细胞的功能。研究发现，沙门氏菌通过释放一些毒性物质来破坏白细胞的正常功能。这些毒素可以抑制白细胞的活动，减少其数量和活性。因此，在伤寒和副伤寒中，白细胞减少可能是由于沙门氏菌对免疫系统的干扰引起的。

此外，沙门氏菌还可以引起炎症反应，导致白细胞浸润到感染部位。在这种情况下，大量的白细胞会聚集在感染部位，导致血液中的白细胞计数减少。此外，由于沙门氏菌的侵袭性和毒力，感染过程中往往会出现发热、恶心、呕吐等症状，这些因素也可能导致白细胞计数的减少。

总结起来，伤寒和副伤寒时白细胞减少的原因可能有多个方面。第一，沙门氏菌的生物学特性使其能够逃避宿主免疫系统的攻击。第二，沙门氏菌释放的毒素可以抑制白细胞的活动。第三，沙门氏菌引起的炎症反应可能导致白细胞浸润到感染部位。这些因素共同作用，最终导致了伤寒和副伤寒时白细胞数量的减少。

In typhoid fever and paratyphoid fever, both caused by Salmonella bacteria, a notable characteristic is the decrease in white blood cell count. So, why does this reduction in white blood cells occur during these diseases?

Firstly, we need to understand the role of white blood cells in the body's immune system. White blood cells are important immune cells responsible for defending against pathogen invasion. When we are infected with bacteria or viruses, white blood cells rapidly increase in number to respond to the infection. However, in typhoid and paratyphoid fever, the count of white blood cells decreases.

Research suggests that the decrease in white blood cells may be related to the biological characteristics of Salmonella bacteria. Salmonella is a Gram-negative bacterium with strong invasiveness. After entering the human body, Salmonella employs intracellular parasitism to evade attacks from the host immune system. This parasitic mechanism allows Salmonella to escape the host's immune response and cause infection.

Furthermore, Salmonella can interfere with the function of white blood cells. Studies have found that Salmonella releases toxins that disrupt the normal function of white blood cells. These toxins can inhibit the activity of white blood cells, reducing their count and activity. Therefore, the decrease in white blood cells during typhoid and paratyphoid fever may be caused by the interference of Salmonella with the immune system.

Additionally, Salmonella can induce inflammation, leading to the infiltration of white blood cells at the site of infection. In this case, a large number of white blood cells gather at the site of infection, resulting in a decrease in the white blood cell count in the bloodstream. Moreover, due to the invasiveness and virulence of Salmonella, symptoms such as fever, nausea, and vomiting often occur during the infection, which may also contribute to the decrease in white blood cell count.

In summary, there may be multiple factors contributing to the decrease in white blood cell count during typhoid and paratyphoid fever. Firstly, the biological characteristics of Salmonella allow it to evade the host's immune system. Secondly, toxins released by Salmonella can inhibit the activity of white blood cells. Lastly, the inflammatory response caused by Salmonella can lead to the infiltration of white blood cells at the site of infection. These factors collectively result in the decreased count of white blood cells during typhoid and paratyphoid fever.