In immunology two types of immune response are described depending on whether there is prior contact with the antigen.
• At this first contact, only lymphocytes with receptors specific to the antigen in question are stimulated and lead to the production of antibodies capable of neutralizing it. The lymphocytes capable of recognizing a specific antigen are very few and the production of antibodies - of low intensity and limited duration - is detectable only after a period of latency of several days. This is called the primary response. The specificity of the resulting seropositivity, however, makes it possible to detect contamination (seroconversion).
• Upon re-contact with an antigen that has already triggered a primary response, specific antibody production is found to increase rapidly after a short latency period. The secretion of IgG then reaches levels much higher than those observed during the primary response, while that of IgM is of the same order. This early and intense response, called secondary response, most often results in the destruction of infectious agents before any clinical sign.
• Many centuries before the discovery of microbes, it was noted that people who developed a contagious disease without death were then specifically immunized against the disease, without being protected against others.
• The ability to react appropriately to an already received stimulus supposes the existence of a memory. The immune memory is based on the existence of memory B and T lymphocytes, resulting from the clonal expansion due to the first contact with the antigen. Their lifespan is significantly higher than that of other lymphocytes and their high reactivity gives its rapid and intense character to the secondary response.
Correct answer: A). The current hypothesis of the evolutionary relationships between organisms
The tree of life is a metaphor that expresses the idea that how life is related to ancestors. Charles Darwin first uses this in the modern biology. It is an evolutionary tree that depicts the relationship between various biological entity. It holds the data that are collected from RNA, DNA and protein analysis. It shows the current hypothesis about the evolutionary relationship among the taxa.
Hence, the correct answer would be option A.
The answer is B cell wall
Chromatin
Chromosome, the highest packing level of DNA, is packed in nucleus. DNA strand itself is packed in chromatin. Chromatin is DNA packaging into 30nm fibers that involve DNA strand with scaffolding around DNA proteins, like histones, in eukaryotic cells’ nucleus.
Explanation:
The DNA strand is folded into nucleosomes, bead-like structures on the DNA and fold up into the 30nm fibers. During meiosis and mitosis, the chromatin is super-coiled up into thicker fibers called chromosomes. Chromosomes within the nucleus can be viewed by a light microscope in a cell in metaphase.
Learn More:
For more on levels of DNA check out;
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