**1. Introduction**

*Chlamydia* genera belong to the *Chlamydiae* phylum, Chlamydia class, *Chlamydiales* order, and *Chlamydiaceae* family according to phylogenetic classification. *Chlamydiae* are Gram-negative non-motile obligate intracellular bacteria with biphasic developmental cycle. They appear in two different forms called the elementary body (EB) and the reticulate body (RB). EB in the extracellular environment infects susceptible cells. After cell invasion, the disulfide bonds of EB are reduced and the bacteria transform into the RB form. This intracellular form, which is responsible for the persistence of the disease, is metabolically active, replicative, and non-infectious [1–4]. Many species of chlamydia occur as disease agents in humans and animals. The two main species responsible for disease in humans are *Chlamydia trachomatis* and *Chlamydia pneumoniae*. In addition to these two species, which are strictly human pathogens, *Chlamydia psittaci*, whose natural host are birds, also causes infection as a result of transmission to humans [5, 6]. *C. pneumoniae* is

primarily associated with bronchitis and pneumonia. Acute infections are usually asymptomatic or mildly clinical. Reinfection is common. In recent years, there has been evidence that *C. pneumoniae* causes some inflammatory diseases. Chronic bronchitis, asthma, atherosclerotic cardiovascular diseases, and cerebrovascular diseases are among the diseases associated with chronic *C. pneumoniae* infection [7]. *C. trachomatis* is grouped into various serovars according to its outer membrane genotype. Serovars A, B, Ba, and C mainly infect the conjunctival epithelium, causing trachoma defects. Recurrent infections can result in corneal damage and blindness [8]. Serovars D, Da, E, F, G, Ga, H, I, Ia, J, and K are causative agents of urogenital chlamydial infection [9]. It causes proctitis and urethritis in men. In women, it causes urethritis, cervicitis, endometritis, and salpingitis. Chronic pelvic pain, ectopic pregnancy, and infertility are common complications, accounting for 20–50% of nongonococcal urethritis cases [10, 11]. L1, L2, L2a, and L3 serovars spread to the inguinal lymph nodes and cause lymphogranuloma venereum (LGV) [12]. Although sensitive tests such as PCR can be used for the diagnosis of chlamydial infections, their asymptomatic nature delays their diagnosis and increases their spread in the community. Sequelae increase in the long term as a result of not applying timely and appropriate antibiotherapy [13]. Chlamydial infections continue to be an interesting research topic due to their impact on public health. The necessity to understand all aspects of chlamydial infections necessitates the examination of host responses. Various stages of the innate and adaptive immune response have been studied in numerous studies, both in humans and in animals. In this topic, immunological events that occur in the host after chlamydial transmission are summarized based on the literature.
