tailieunhanh - Introduction to Medical Immunology - part 4

Trong vài giờ sau khi nở, có một giảm nhanh chóng trong các kháng nguyên bề mặt, như được phản ánh bởi sự suy giảm tiến bộ trong sự gắn kết của tinh khiết và fluorescein nhãn hiệu dị kháng thể antischistosomular tới màng tế bào ký sinh trùng. | Page 250 1. In the few hours after hatching there is a rapid decrease in the surface antigens as reflected by a progressive decrease in the binding of purified and fluorescein-labeled heterologous antischistosomular antibodies to the parasite membrane. This is observed even in parasites cultured in serumfree medium. 2. Schistosomula recovered from infected animals have host proteins bound to the outer layers masking the remaining parasite antigens. E. Antigenic Variation has been characterized in bacteria Borrelia recurrentis parasites trypanosomes the agents of African sleeping sickness Giardia lamblia and viruses human immunodeficiency virus HIV . 1. African trypanosomes have a surface coat constituted mainly of a single glycoprotein variant-specific surface glycoprotein or VSG for which there are about 103 genes in the chromosome. At any given time only one of those genes is expressed the others remain silent. For every 106 or 107 trypanosome divisions a mutation occurs that replaces the active VSG gene on the expression site by a previously silent VSG gene. The previously expressed gene is destroyed and a new VSG protein is coded which is antigenically different. The emergence of a new antigenic coat allows the parasite to multiply unchecked. As antibodies emerge to the newly expressed VSG protein parasitemia will decline only to increase as soon as a new mutation occurs and a different VSG protein is synthesized. 2. Giardia lamblia has a similar mechanism of variation but the rate of surface antigen replacement is even faster once every 103 divisions . 3. Borrelia recurrentis the agent of relapsing fever carries genes for at least 26 different variable major proteins VMP which are sequentially activated by duplicative transposition to an expression site. The successive waves of bacteremia and fever correspond to the emergence of new mutants which for a while can proliferate unchecked until antibodies are formed. 4. HIV exhibits a high degree of antigenic .